We occasionally hear requests to be able to remove or edit messages on Upwork and we appreciate the suggestion. However, it is not something we will be able to provide in the near future as messages become very valuable when it comes to disputes or any other issues between client and freelancer. Currently, a message thread can be completely removed only by our engineers. In your case, I would just suggest apologizing to your client for the mistake, I am sure they will understand.
I believe what you said that it is better not deleted so it can be researched in times of dispute, but at least I hope there's an option to archived it. Or perhaps an option to create a folder to organize messages. Like in my case, I have messages that I don't want to see and I hope I could be able to see the active messages or those I need only and maybe archived or move in another folder those I don't need anymore.
Come on, there will come a time when you really need to delete messages. Just give the responsibility to the freelancers/client to keep messages which they think they will probably still need in the future.
I'd recommend a way to archive/hide all messages
I go through every message and click hide message as they are all old, but then every time I open the screen the same messages pop up. It is really distracting, these are all old messages. I'm seeing the same messages pop up from weeks or days ago and it's really distracting if you are working on a proejct.
Why are they still there? I click on unread and still there are old messages on the right of the screen.
I now just hid all messages and then they come up again.
It is very aggravating and I wouldn't use your website but I don't have any other way to find people.