Forum Discussion
Restricted domains sharing at site collection level - no longer an option?
Directions to configure domain-restricted sharing at the site collection level was outlined June 2018 at https://docs.microsoft.com/en-us/sharepoint/restricted-domains-sharing?redirectSourcePath=%252...
I still have it, but not in the modern Admin Center...it's in the classic one so as advised by Vasil you will need to configure it through PS