@Tim Lumb Apologies for the delayed response, Regarding your ask for "Release date for Microsoft Entra custom claims provider".
Checked internally, this feature - Microsoft Entra custom claims provider is in GA (Generally available), same will be documented in the release notes of April 2024 - https://learn.microsoft.com/en-us/entra/fundamentals/whats-new
Let me know if you have any further questions, feel free to post back.
Please remember to "Accept Answer" if answer helped, so that others in the community facing similar issues can easily find the solution.