New version of lib is available on github or nuget.
https://github.com/mariuszkerl/AspNetCoreSubdomain

Now you can constraint subdomain same way as you would do constraints on normal routes. More info here:

https://github.com/mariuszkerl/AspNetCoreSubdomain/wiki/Constraints

If you wonder about use case imagine having test enviroments which you generate, each of them is represented by guid, now you can host it on a single app and single domain, constraining subdomain as a guid. There are some restrictions for constraints which are specific to domains, all you can read on above wiki page.