Maybe to give a concrete example:

When /r/anarchism (left leaning sub about anarchism, antifa, communism, ...) mods refused to enforce reddit's rules relating to the incitement to violence, reddit only removed the mods in question instead of closing the sub.

When /r/physical_removal (right leaning sub about pinochet 'memes') mods refused to enforce reddit's rules relating to the incitement to violence, the sub was removed.

When /r/nazi (third position leaning sub about national socialism) - well, I don't actually know whether the mods there refused to enforce reddit's rules. It's been removed.

/r/nazi was removed because it promoted Nazism.

I think left "leaning" is a bit understated.

Is there an upper bound on "leaning"?

Leaning just implies that they aren't completely fallen over to one side, which I think is very much the case.