theoretical draws

[syzygy]

10 light bishops is an illegal position

Not illegal but unreachable from the starting position ... Therefore few GUIs (including CB) do not allow to enter such a position graphically.

A position is illegal when it cannot have been reached by any series of legal moves.

(Article 2.3 specifies the initial position.)

So, according to FIDE, "legal" and "reachable from the starting position" are the same.

Unfortunately, reachability as a criterion for legality is too complex to calculate to be very useful.

(Somewhat similarly, I think current Article 9.7 ("The game is drawn when a position is reached from which a checkmate cannot occur by any possible series of legal moves. This immediately ends the game, ...") is not a good rule. For example, it means the game ends here (not by agreement, but it simply ends full stop):
[D]8/5b2/2k5/8/6B1/8/8/Kr6 w - - 0 1
That's just silly imho.)