One notable limitation of the Shapechange spell reads as follows:
You transform into an average example of that creature, one without any class levels or the Spellcasting trait.
However, True Polymorph states no such limitation. Does that mean that you could turn into a creature with the Spellcasting feature? Some examples of such creatures would be the Drow Matron Mother, Archmage, Archdruid, etc.
If this is permitted, does it mean you could make creatures with the Spellcasting feature that don't normally have it, provided that you stay within the CR restrictions? For example, could you create a Nothic with Wizard levels?
Best Answer
Yes, you can true polymorph a creature into a spell casting NPC
The rules text is pretty clear about this.
If you want to True Polymorph a level 12 or CR 12 creature into an Archmage NPC, CR 12, you can do it, and it will be able to cast its spells.
{Bolding mine, spell list edited out for brevity}
See this answer for a detailed breakdown of what's in a creature's stat block.
If you look at the Nothic's stat block, you don't find the spell casting ability, so that CR 2 creature would come as is - without spell casting.