So, as I go through the list of things that I like about 5e as compared to 1e, and things I like much, much better in 1e the whole concept of wearing armor and casting spells comes up. In 5e, this is simply a matter of proficiency – if you are proficient in the armor, then it doesn’t interfere in your spell-casting. Now, in 1e spell-casting and armor was severely limited and was one of the great balancers for non-human races, fundamentally for Arcane Magic.
Now, truthfully, there are all sorts of different flavors of Arcane Magic now (and we’ll ignore my “historical game” switched all sorts of things up, like Druids using Arcane Magic, blah, blah, blah…) but, in the quest to nerf the idea of level-dipping, and continue to add back at least some of the verisimilitude that made my campaign world make sense…
Divine Magic has no inherent limits on armor (just like 1e), it is simply a matter of the armor training you get from your class. A character Deity is happy to pump divine energy into you, whatever you’re wearing, as long as you’re doing “the right stuff”!
Arcane Magic is where it gets wonky…
Wizards, Eldritch Knights, Arcane Tricksters, and Sorcerers may only wear only wear Ultralight Armor.
Bards and Warlocks may wear Ultralight and Light Armor.
Elves, High Men, Half-Elves, Sh’dai, Dwarrow, the Old Race, and Gnomes (this could expand as additional races are detailed) may wear non-metallic Light and Medium Armor and cast Arcane Magic, they may also wear enchanted metallic armor of the same types.
This gets us back to the image of locking wizards into specially-made suits of armor as a way to neutralize them without having to cut their tongues out or cut off their fingers and hands… It’s also the reason why these races are likely to get targeted first by tactically knowledgeable opponents, they are going to be assumed to be spell-casters, no matter what they actually are, and are perceived as mysterious, dangerous, and the most significant threat sans any more obvious target.