SIMBL Agent crashing frequently

I just replaced a MacBook Pro running High Sierra with a new MacBook Pro M1 running Big Sur, and migrated my applications and accounts using Migration Assistant.

I noticed this evening that ReportCrash is using quite a bit of CPU, sometimes as much as 25% in both my regular user account and my admin account. I opened Console and saw that there were lots of Crash Reports from SIMBL Agent -- every 20-30 minutes in my user account and even more frequently in the admin account.

I removed the directories ~/Library/Application Support/SIMBL and /Library/Application Support/SIMBL (they both just contained SafariSource.bundle), but this didn't solve the problem.

Then I checked system.log, and every few seconds it's logging

Aug 29 00:11:05 Barry-Margolins-MacBook com.apple.xpc.launchd[1] (net.culater.SIMBL.Agent[29138]): removing service since it exited with consistent failure - OS_REASON_OBJC | Objective-C garbage collection is no longer supported.
Aug 29 00:11:05 Barry-Margolins-MacBook com.apple.xpc.launchd[1] (net.culater.SIMBL.Agent): Service only ran for 0 seconds. Pushing respawn out by 10 seconds.

I tried killing this launchd service with

sudo launchctl unload /Library/LaunchAgents/net.culater.SIMBL.Agent.plist 

and got the error:

/Library/LaunchAgents/net.culater.SIMBL.Agent.plist: Could not find specified service
Unload failed: 113: Could not find specified service

What do I have to do to kill this?

I removed the file

/Library/LaunchAgents/net.culater.SIMBL.Agent.plist

and rebooted, and this has stopped.