Alerting for macos-bbw2 worker is constantly reporting > 90 % disk usage on FS /System/Volumes/Data/
After some research I've found that a possible culprit is Spotlight.

SpotLight is building an INDEX for every file written on the FS, under normal operation it should reclaim space when the file is deleted. Spotlight indexing is usually used for Finder or other utilities that need to "find" very fast a file on the drive.

It seems that the nature of Buildbot of writing tons of small files under longer periods is not letting Spotlight clear it's index properly.