I have just recently noticed that there is a MAX_SKILL field in the game (here). The nearby comment seems to imply that skills > 10 will perform identically to skills = 10.
Does this constant actually do anything? Should I turn off skill gains for level 10 skills to preserve focus? Or is this just a constant that does nothing in practice, letting skills (and benefits from those skills) increase arbitrarily high?
Yes. There are uses of it; it limits the max speed at which you can (un)install vehicle parts for example.
However, it’s not a hard cap if I understand that right. You might still profit from higher skill levels (with diminishing returns, of course) in different fields, like (currently) disarming traps and melee.