Which makes T9 fail miserably because it works with a non-abbreviated words.

Later there were adaptive versions which had an auto-populated user dictionary but that made "a blind T9ing" prone to errors.

I want to say you could add custom words no problem? But it's been a hot minute, so I might be misremembering.

Meh, Imma team Moto and I don't remember it well anymore, 20 years give or take.

Yes, the later variants both had a custom/user dictionary and could learn %he new words from the input. The latter could add the uncertainty in the input.