If you reboot (I know, such a Windows term), for sure it won't start the renamed bitcoind again.
I'm trying to save rebooting as a last resort as it means I have to manually start up all my services and workspaces.
Update:
It appears to be caused by an Ubuntu service called
How do I stop this service? systemctl stop/disable/mask apport don't seem to be working.
SOLVED.
That bitcoind instance was not running directly on my system, it was running in a docker container as part of a leftover and long-abandoned BTCPayServer installation. That's why killing it kept respawning the same process. So it actually had nothing to do with Apport or whoopsie.
Running sudo btcpay-down.sh followed by sudo btcpay-clean.sh did the trick. Saved me from an unnecessary reboot (BTW that probably would not have helped anyway, since BTCPayServer Docker containers autostart with the system).