Backups are not physically deleted from Server Backup repository when domains, customers or resellers are removed from Plesk.
This is Plesk internal issue with ID #PPPM-4396 , which is planned to be fixed in future Plesk updates.
Delete backups of removed domains, customers or resellers manually from the directories below:
213904665 [HOW TO] Find and change default server backup repository
213905625 [Info] Backup structure in Plesk