I tried viewing Notion and self-hosted Mattermost using SeaMonkey last week. Nothing was displayed at all. Trac pages still worked though. So yeah, a web browser needs more than security fixes.
Start blaming the sites for using new Chrome(ium)-exclusive features when they're not otherwise necessary. The fact that many sites which decide to update and then offer zero or often negative value to the user, while requiring a newer browser, should be proof that Google is doing an embrace-extend-extinguish on web standards, and web developers who get sucked into their propaganda are making it harder for alternative browsers.