Thank you for reaching out to us. Happy to help. I reached to internal team to check and confirm on your ask. Here are the details:
possible to reassign the name (owned by Microsoft) to a new service
This isn’t possible unless you migrate your existing cloud services (classic) to extended support using In-Place migration path. Only then we enable users to retain their DNS FQDNs assigned to the IP endpoint on the cloud service.
For your question on .net5 support, we are not enabling any new capabilities with extended support. Given this isn’t supported on classic today, and as of now this is not on the extended support roadmap either.
Hope this helps.
Please 'Accept as answer' if the provided information is helpful, so that it can help others in the community looking for help on similar topics.