This came up on another thread.
The idea that a person in heavy armor is incapable of doing anything and is weighed down so much they need assistance to mount a horse. There's videos of people doing cartwheels in plate, I had a video at one point (lost the link) of a middle-aged guy in heavy chain swimming without an issue because the gambeson you wear underneath basically acted like a flotation device. Add in only people in heavy armor are affected by heat (while never having protection from cold of course) Pretty much any armor of any use historically had a thick layer of padding underneath; any armor that will realistically gives you any significant protection will be hot.. Sleeping in heavy armor (but breastplate is perfectly okay, as is "studded" leather) means you're exhausted but everyone can sleep just fine sleeping on hard rock*.
Meanwhile all those 8 strength PCs can be at 99% encumbrance with absolutely no penalty. D&D is not particularly realistic about much of anything, but the moment someone dares run a PC that isn't dex based for AC they're practically immobilized because it's deemed more "realistic" by people who have never worn properly fitted armor.
*I used to backpack a lot and never brought a sleeping pad, I never had a problem sleeping. I have no idea how bad it would be to sleep in armor, but it is fitted to you. The only times I had problems sleeping was because of wet and cold or extreme heat.