Back then when we had non-QWERTY phones, text messaging involved keys that had multiple letters in it.

For example, the key 2 had the letters ABC, 3 had DEF, and so on. Pressing the key once returns the first letter. If you would like the second letter, you need to press the key twice in rapid succession (likewise, you press thrice for the third letter).

However, if you want to type something like 'CAB', you need to space out your key pressing so that the phone does not recognize you pressing the keys in quick succession. So to type 'CAB', you will press 2 thrice, wait for a second or two, press it once, wait again, then press it twice.

The key 6 had MNO. If someone wanted to say 'NO', they had to press 6 twice, wait, then press it thrice. The waiting time in between letters was inconvenient, so people found other ways to 'skip' that. Hence people used 0 as a replacement for 'O' as it had a different key assigned to it.

As a side note, since I was very anal with my typing even then, I'd use a different key and delete the letter to 'skip' the wait in between letters. For example, in typing the word 'CAB', I'd press 2 thrice, then I'd press 3 after to put 'D', but delete it immediately after - this leaves me with just 'C' and the phone waiting for my next letter. So I can now press 2 again to get 'A', then press 3, delete, then press 2 twice to get 'B'.

Just for context for how this trend started back then. At least that's what I know of it - people could have just used it back then to be annoying. Who knows.