I remember disabling T9 as the first step when getting any "new" phone, so it must've been enabled on some. Then again, I rarely got phones new from the store, so this might have been because they were factory reset.

I don't think I ever really knew what it was, other than "that thing that made typing difficult". I doubt any phone shipped with a Slovene dictionary for T9, so it was probably just doing its best trying to map my seemingly nonsensical keypresses onto English words.