I'm fairly certain it's a default USE flag, but I'm an idiot who likes to ignore such things. My system is currently built with somewhat minimal USE flags. The USE flag stack-align was disabled because of the supposed performance costs of leaving it enabled. Turns out with it disabled it seems Steam can't start any games. No proton games, no native games, nothing. Soon as the flag was turned back on, games started working again.
STEAM_RUNTIME on or off doesn't seem to make a difference, either.
Yeah I doubt there is many people this would affect but I figured if Steam isn't going to work with it off the ebuild should force it as a dependency for ding dongs like me.