can you delete old Mac backups

Are you referring to Time Machine backups? If so, the short answer is no. Time Machine will do this automatically when it senses the Time Machine External Hard Disk is getting too full. NEVER EVER manually remove Time Machine backups or you will likely make Time Machine unusable.

Is the Mac that you want to delete old Time Machine (TM) backups running macOS Big Sur? If so, then as @rkaufmann87 has mentioned, TM no longer provides you with a means to delete backups. This has to do with how TM now works with the APFS format.

For earlier versions of macOS, there are still a number of methods of deleting backups, both within the TM interface and outside of it. The latter having a greater potential to corrupt the backup set.