It is commonly used informal verb in the UK. As well as meaning 'tired' it's another way to say that something is so tired that it's actually broken: eg. 'My car is knackered' (my car is so damaged / old that it's broken or 'I knackered my knee playing football on Sunday'. You can also use as a phrasal verb with the word 'out': eg. 'you knackered me out' (you made me tired).

As far as I know it's still as popular as ever in the UK. It's more than just colloquial, though, and would be considered vulgar by many. According to Google Ngram Viewer, its use hasn't declined much recently. Why are you under this impression, by the way?

I have heard it. Not so widely used in Australia. Mostly used in the UK.

Never used in the US. Commonly heard from UK media

I hardly use it and have not heard it for a long time except with the older generation. It means being tired. Most people will just use a word like exhausted and just say they are really tired.