Pretty sure copy/paste was removed by design to limit some chat abuse. It was possible in the past.
Chat could be better
I also think that the feature should be allowed (or improved). Because I am not good at English, I use Google Translate, but it is not available in chat.

Pretty sure it's possible if you do it in a certain way because I do it constantly to translate non-english
So when you are in the chat thing for texting you can't highlight to copy and paste. I think that if this got fixed it would be more user friendly. For example, I sent my friend a website link and he couldn't copy and paste it. This can only be done in the messages.
Just a recommendation.