How to disable event notifications for group I'm not in?

In your friends list, there's two tabs - one for friends, another for groups. Click the groups tab, and you'll likely see the tomb raider group there. You can remove yourself from there.

If that doesn't work, go to the Tomb Raider community hub. There should be an unfollow button on the page:

Between these two locations, you're guaranteed to never be notified of any Tomb Raider events ever again.

Apparently, with the new "Explore your Queue", when you choose to follow the game, it adds that game to your groups and it can only be removed through the community hub of that game.

Do not follow games if you do not want notifications from that game.

You only need to open STEAM. Click "STEAM" in the top left corner, then click "Settings", this opens the setting window. Now click "Friends" and near the bottom locate "For Group events and announcements" and uncheck "Display a notification". This will stop the issue.

Go to your name on Steam and click "activity" in the dropdown menu. On the top of the next page, click "friend activity settings" and from there you should be able to turn off all the popup notifications you don't want.

I found it! As of 2020 Steam. Open Steam, click your Name under the bar:

[Store Library Community Your Name] (Takes you to "activity")

Then on the bottom right you'll see View Groups, and even Upcoming Events including the ones you are sick of seeing.

Click View all X groups.

Find whatever group won't stop spamming you with events and click Leave Group.

Open FRIENDS > CLICK SETTINGS (gear icon) Then you'll see Group Events and Announcements at the bottom of the list: