The great thing about those keyboards was that a lot of people could just turn off text prediction and type text with a single hand without looking at the screen.
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The great thing about T9 (certainly on the Nokia 3210 and 8210) back in the day was you could type messages fast with few k/s without looking. As long as you had enough experience to know the word combos.
Ive never found a t9 system as good as the Nokia implementation. In some respects its better than qwerty for short messages. And don’t get me started on apples fundamentally broken auto correct system. People dont know any better these days. There’s actual adults walking today that have never typed on a real keyboard.
Later there were adaptive versions which had an auto-populated user dictionary but that made "a blind T9ing" prone to errors.
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.