In addition to the link you provided, this one also mentions the removal, but provides no reasoning or justification, which only makes the situation all the more frustrating as we watch the investment in our custom providers go to waste:
What's new in ArcGIS Server 10.8—Documentation | Documentation for ArcGIS Enterprise
Custom identity stores are a very valuable feature that allow us to link map service permissions to application permissions. In this way, we only have to maintain a single ACL. Without it, we have to maintain duplicate ACLs: one for application permissions, and the other for map service permissions because the latter does not imply the former. This alone is an administrative headache, but these ACLs will undoubtedly get out of sync which only compounds the problem, especially if you have to manage multiple, separate installations.
Unless these are brought back, developing some automated tool that translates the application ACL to ArcGIS Server via the REST API on a periodic or ad hoc basis is about as good as it's going to get...which is too bad, because it's already been proven that it could be so much better.
