Minecraft Commands – How to Remove a Specific Trade from a Villager in Minecraft 1.16

The exact mechanics of how a villager generates their next trade are, from the link you posted:

The game first compiles a list of potential offers, and the probability is used to determine whether the offer is included in this list. Then one offer is selected from this list at random to be the actual new offer generated.

So basically, every trade that a villager legitimately has in their list of possible trades, has a chance to go on the list. After the list is built, a single item is chosen from among those that made it onto it, and the chosen item is the villager's new trade. There is always, however, the chance that a villager will generate a new trade, and the list will be empty; when this happens, the villager gets the Gold Purchase trade you're after.

For some types of villagers, this is really unlikely, but priests have a fairly good chance of it, because they only have a dozen possible trades, every one of which is more likely to be off the list than on.

Now I'm not a genius when it comes to command blocks so I can't tell you what is wrong but I remade your command and tested it in 1.9.2 (which worked) and by comparing my command and your command there is some differences.

/summon Villager ~ ~1 ~ {CustomName:"The First Wizard",CustomNameVisible:1,Profession:1,Career:1,CareerLevel:42,Offers:{Recipes:[{rewardExp:0b,maxUses:2147483647,uses:0,buy:{id:emerald,Count:1},sell:{id:potion,Count:1,tag:{display:{Name:"Super Potion"},CustomPotionEffects:[{Id:5,Amplifier:5,Duration:600}]}}}]},Attributes:[{Name:generic.movementSpeed,Base:0}],Invulnerable:1,PersistenceRequired:1,Silent:1}

The generator I made this with is

http://mcstacker.bimbimma.com/

It might be a bit confusing at first but it helps me make several of my commands. It even gives a description for all/most of the options it gives.

Hopes this helps!