When Time Machine backs up to HFS+ you can clean up those backups by deleting working folders and files.
Despite its strengths, when Time Machine in Big Sur backs up to an APFS volume, you can no longer change backups, only delete a complete backup. However, excluding your working folder from backups altogether would mean you’d need to make alternative arrangements, perhaps using Carbon Copy Cloner (ML186, p80) to make backups of your work in progress.