Something that D&D 5e allows. Spellcasters can wear and cast spells in armor if they are proficient with that category of armors (light/medium/heavy). While the Wizard class doesn't give any armor proficiencies, the Shield Dwarf subrace gives proficiency with Light and Medium armors.
Another common route to get a medium/heavily-armored caster is to multiclass into or start your character with 1 level of fighter (eventually getting to 2 levels for Action Surge) which grants armor proficiencies. Multiclassing isn't (yet) in BG3 though, though we haven't heard Larian say they're *not* going to implement it...
