I wanted to open Slack today and saw this message. I have the latest version of Opera and installed the latest update yesterday. Yesterday everything was fine. Opera is actually not on the list of browsers supported by Slack.
@rafalspuka hey, we are handling this by overriding user agent on site. This should be handled by siteprefs.json file in your profile. Could you check if you have it available? Information about your profile folder location can be found on opera://about page.