At the time we set up the integration/composer git repo because there was no Debian package and we wanted to use a relatively recent version. The version of composer in buster+ is recent enough (https://tracker.debian.org/pkg/composer) and I think switching to the packaged version would help us get rid of one more oddity. This is blocked on Toolforge fully moving to buster AIUI.
This will also take care of T249949: Stop using integration/composer and then archive the repo.