@miserableget: Google Chat appears to be a Gmail feature, therefore you may wish to check your notification settings via Settings > Apps > Gmail > Notifications, and disable some non-essential options such as 'Miscellaneous'. If you don't use Gmail, disable the notifications completely and this should resolve the issue.
@miserableget ,Hello, if it is a silent notification, try this. Slide the notification aside, you will see the small adjustment wheel, click on it. A window similar to the screenshot appears. Turn off both switches and confirm. Greetings.
I had started to get pop ups that just flashed up for a millisecond (instead of the ones I had to click away) and and deleted whatever I was writing in e.g. Messenger. Extremely annoying. I was (again) in contact with Samsung support and finally found a guy that could help. He asked me to try and 'force stop' for RCS. I went to 'Settings' (the general ones for the phone), then 'Apps', then had to click on a 'sign with arrow and 3 lines', to the right of the list. Then chose 'Show System apps', then 'OK' and finally Find 'RCSsettings' in the list. When opened click the Force stop at the bottom.
Not sure my translation from Swedish is correct, but hopefully you can figure it out. It helped me at least. No more pop ups since yesterday!!!