I have not tested this extensively and it may turn itself off occasionally (though it hasn't yet for me), and I don't know if it's synchronized to all devices, but it is in a file that is synched with Steam Cloud so it should be. Obviously a control for the setting is incoming at some point (just like for group events & announcements), but until then...
Close the Steam client.
Ctrl + F (
Cmd + F on Mac) and search for
"bCategorizeInGameFriendsByGame\":true, and change true to false.
Save the file and reopen Steam.
Found while looking through the configuration data as I'm currently working on updating the Steam-Data project, which is an attempt of Reddit/GitHub user ObsidianMinor and myself (and anyone else if you'd like to send a pull request, file an issue, or etc) to document most of the Steam client's local configuration data (trying to leave unknown/unsafe data out of it).
Copy/pasted from the reddit post I made.