My candidate: shutting down Google Hangouts a few months before Covid hit. They lost the generic brand name to Zoom. Before that it used to be called "a hangout".
I'm told it has resurfaced as something called Google Meet.
Microsoft's equivalent (of losing to Zoom, not their worst decision) would be buying Skype for $8.5 billion when it wasn't worth that much, changing features that made the beloved brand unusable for years, then rebranding it as Teams and forcing me to see that never-used logo in the bottom right corner of Windows. Never used it, never will, want it gone, but I can't. On the upside, I've heard that Teams is usable, and a worthy competitor to Zoom.
What's your candidate?
From mine? Killing Google Reader. This is when something changed about Google in the minds of many.