Casting Spells in Armor:
No. Just no.
Yes, elves could in passed editions. Why? Cuz they're elves! Inherently magical beings. They could do (along with bunches of other stuff) things the other races couldn't.
If you had elfin chain...again, elves made it, inherently "ok with magic beings making armor that's ok for casting...for themselves." Makes sense. You couldn't buy it. You had to find it. It was supposed to be extremely rare...even if you were an elf!
How this becomes "Well, elves could, so everyone should be able to" is just so much more of the entitlement crowd wanting to have their cake and eat it too.
A dwarf in plate mail with a hammer in their one hand...using arcane magic? I don't know what that is...but it's not a D&D mage. A human in scale mail, carrying a shield and casting arcane spells? Again, dunno what it is...maybe a Final Fantasy something or a WoW bastardization...not a D&D magic-user/mage/wizard...or even sorcerer or warlock.
Back in 1e & 2e days, we also had multi-classed fighter/mages (or cleric/mages or thief/mages) who could cast arcane spells if they were wearing magic armor...the armor itself infused with magical energies so they didn't disrupt spellcasting. Made sense. Still does. Now, if that was actually in a manual somewhere or we just houseruled that way, I can't quite recall at the moment.
I'd also like someone to clarify for me alllll of these so many character concepts that should be permitted to cast arcane spells in armor?
There's the guy who's in-your-face-with-a-sword combatant who also knows some arcane magic...annnnd? That's it. There's the concept. Call it a "spellsword", "bladesinger", "eldritch knight" or a hundred other things...the archetype is the same: a hybrid/multi-classed "Fighter-Mage." Soooo...who're all of these other oh so integral character concepts that the game simply must allow armored arcane spellcasting?
Do you think the final game will change this?
Sadly, no. I do not. Those that think a dwarf in plate armor with a war hammer or a chain-clad halfling with a shield is a necessary arcane spellcaster character concept for the game to accommodate will be able to have their armored cake and eat their spells too.
Now if you'll excuse me, I have some grass to yell at to get offa my lawn. Then think I'll lament the blueness of the sky.