Lots and lots of software gets worse over time. In fact, that may be the default state of software.
I don't think I've seen as plainly visible a decline as I've seen in Doodle, though. I started using it around 2016 or 2015 and have since been using it once every two to three weeks to schedule D&D games.
It used to just do the thing you wanted it to do: Gather consensus on when a meeting should happen. But starting a few years ago, they started adding extraneous features making it harder to find the essential stuff, then increasing the bugs. A bit later, they started pushing features I used, like duplicating old invites, to the pro tier. Which I might have paid for, if my trust hadn't been eroded.
Today, creating invites has stopped working in Firefox.
Looks like they were acquired by an AI-oriented company in 2016, so maybe it was doomed throughout my entire time with it. Then in 2019, that parent company was shut down. Anyway, you can very easily imagine the meetings about product vision and OKRs every time something breaks.