Does it support LDAP/Active Dir authentication?
OpenIddict doesn't handle the user authentication part. You're free to use whatever you want, Windows Integrated Authentication (NTLM/Kerberos) or manual LDAP lookups included.
Is there an example how to include it?
No.
@auroris created a very nice sample showing how to use Windows Integrated Authentication with OpenIddict 3.0's degraded mode (on IIS): https://github.com/auroris/OpenIddict-WindowsAuth
The sample doesn't work the second time round. For example; I enter wrong credentials, I do get 401. I then enter the correct credentials, I get 200. However, I can enter wrong credentials again and it will always return 200.
Most helpful comment
@auroris created a very nice sample showing how to use Windows Integrated Authentication with OpenIddict 3.0's degraded mode (on IIS): https://github.com/auroris/OpenIddict-WindowsAuth