There are a number of different strategies one can take (besides the possibly obvious: remove material spell components/focuses/holy symbols/etc.):
- Make the jail special
Suppress sound (e.g., permanent or ongoing silence) to eliminate verbal spell components
Suppress light so that the spell caster cannot, for example, target individuals or locations via line-of-sight (e.g., use illusions over doors or windows to block line of sight).
Suppress divination (e.g., permanent or ongoing nondetection) so that the spell caster’s buddies cannot scry in order to pop in via, say, teleport, and rescue them.
The jail is built in an area that "naturally" suppresses, hampers or distorts magic.
The jail is constructed using permanent or ongoing anti-magic spells
NOTE: Following up on one of SevenSidedDie’s comments on the original question, spell caster-proof jails may be rare enough and expensive enough things, that one would not expect small towns, or even necessarily any particular city to have the capacity to hold a spell caster (although this will depend somewhat on how common spell casters are in the campaign setting, for example, if every small town has a few high level casters, then these kinds of jail enhancements might be common). This kind of scarcity might make the possession of a spell caster-proof jail a special resource for a region, or even between nations/principalities/city states/etc. where extradition treaties, rendition treaties, or even trade treaties are negotiated around arranging the incarceration of a spell caster (imagine a side narrative about arranging the transfer of a incarcerated spell caster: fun!).
- Alter the incarcerated spell caster
Magically compel the spell caster not to use spells while incarcerated in jail (e.g., a geas, some really nasty custom curse, mark of justice, etc.).
Make the spell caster wear some difficult to remove (probably homebrew) magic item that suppresses somatic movements, sounds, or even magic.
Tag the incarcerated caster with difficult to remove tracking and summoning magic, and make them aware that there will be sanctions for jailbreaks.
Place the spell caster in some kind of suspended animation (e.g., the vanish sink spell of 1st edition AD&D).
Drug the spell caster so that they are incapable of casting (per orlp’s excellent comment).
- Have the jail include guardians who are practiced at containing spell casters.
Abjurationist wardens
Mage-slayer wardens (perhaps equipped for and specializing in non-lethal combat and restraint)
Wardens equipped with anti-magic devices
Wardens that are magic-suppressing, magic resistant, or magic immune monsters or constructs.
- Make compelling appeals to the incarcerated spell caster’s reason, morality or emotions
Threaten the spell caster’s familiar, family, colleagues, or benefactor with sanctions (e.g., your family pays a huge fine if you jail break; your familiar will be destroyed if you jail break; your teacher will be stripped of standing in the college of magic if vouchsafed spell casting students jail break; your first born child will receive the spell caster’s sentence if they jail break; etc.).
Appeal to the sanctity of law (if that is something that the spell caster cares about).
Escalate the legal threats against the spell caster by compounding an original crime with a jail break (e.g., the punishment originally was 6 weeks in the clink and a small fine, now it is the amputation of your right hand).
Make plea deals, so that publicly the spell caster is seen as punished, but the powers that be arrange to trade benefit for leniency in actual sentencing.
A Variety of Suggestions
I have a couple remarks about the starting equipment and saving throws.
Equipment
For one, I see where you're coming from offering both ring mail and chain mail since chain mail has a strength requirement, but chain mail is just a straight upgrade to ring mail provided you have the strength to wear it. If you don't, then odds are you have at least a positive dex score, so scale armor would be a much more appropriate starting armor, modeling from the cleric starting armor.
Secondly, even if they are supposed to be a melee focused class, even barbarians and paladins are given throwing weapon options for a ranged capacity in combat, so think about offering some handaxes or javelins. I would offer them as a choice to the one weapon they get and offer another choice of weapon, or just give some for free.
And the last point about equipment is an apparent oversight. It seems like a class with heavy armor proficiency should have access to shields, which need to be included in the section about armor proficiency. It could of course be a design choice, but I would strongly suggest offering the option, and further including it in your choice of starting equipment.
Saving Throws
Concerning saving throws, having both constitution and wisdom as saves goes against the convention of having a "common" saving throw (dex, con, wis) and a less common saving throw (str, int, cha). Every other class follows this convention, so you should pick one of the two to change into a less common saving throw. Perhaps if you feel they are both thematic you could offer a class feature like the rogue's "Slippery Mind," but keep in mind that's a 15th level feature. Monks are an exception with their "Diamond Soul," giving them proficiency in all saves at 14th level, but I would be very careful handing that out.
Skills
Another miscellaneous comment on skills: it seems like you really are going for a utility based class that's about understanding things and having an eye for detail. I would suggest offering perception and insight as options for class skills, maybe even offer a choice of 3 like rangers.
Class Features
Next, I can offer some editing to the semantics of some of your features. In "Step between the strings" you can replace "unobserved area" by saying "total cover or heavy obscurement." Note this naturally includes invisibility.
The main class features seem fine, but I also have some remarks on the subclass features.
On "Laws of Defiliance [sic]" (which I think should read "Laws of Defiance") it seems that the ability applies to all wisdom saves, but it actually does not affect anything other than a spell. I think it might be ok if it only affects spells, so you should change the wording to say "subjected to a wisdom save by a spell or other magical effect." Any wisdom save at all would be pretty strong, as that includes fear and a host of other abilities.
"Eyes of Death" is probably too strong. An always on truesight even for an 18th level ability should probably require activating with an action, perhaps lasting for an hour, and maybe allow con modifier uses per long rest.
Conclusion
Overall, it seems like a really creative class. I would maybe write a little "quick build" section like other classes on how to distribute stats i.e. "You should pick strength or dexterity as your highest stat, constitution as your second highest stat, lastly wisdom."
Best Answer
I don't think there is anything in the rules about this. But, in addition to the portion of the spell you quote, you can extrapolate from known monsters: Creatures with Int 1 include frog, seahorse, various giant forms of vermin (although the giant fly rates a 2), and similar.
But I'd also consider this:
There's nothing about penalties to one check due to low abilities in another — someone can have a very sickly Con of 1 but still be the World's Best Athlete with Dex and Str of 20 each. It seems that Feeblemind targets very specific parts of the mind concerned with language, communication, and personality — leaving perception and insight unaffected.
A whole village of feebleminded victims with no caretakers would probably have a hard time functioning for a long time — and maybe 30 days is a stretch — but it's not unreasonable to expect animal-level functional survival, given that creatures with Int 1 do survive on their own, plus what the spell itself says, plus the description of actions powered by Wisdom.