Can you delete a migrated photo library

Are you asking about deleting a migrated iPhoto Library or Aperture Library, once you have anew Photos Library?

Yes, if your Photos Library is working correctly and has all pictures and videos you can delete the original library. Check the new library thoroughly, if there are any problems.

The Photos Library does not need the iPhotos Library it has been created from. But I would keep a backup copy on an external volume, just in case you notice any problems later.

You will not save much storage by removing the migrated library however (Photos saves disk space by sharing images with your iPhoto or Aperture libraries - Apple Support). The photos in your new Photos Library are sharing the storage with the photos in the original iPhoto Library. If you did not yet upgrade to Catalina and iPhoto is still running you can keep the iPhoto Library around and open it in iPhoto. Just change the filename extension back from ".migratedphotolibrary" to ".photolibrary". Then iPhoto can open it again, if you are running a system version, where iPhoto is still running.