I was wondering if SQL Server Always On Availability Group supports a single Domain Controller server as opposed to 2 Domain Controllers in every example I've seen. I ask because I suspect that this might be a problem with my HA installation of SQL Server where I use a single Domain Controller.
It depends on what your goal is.
Technically, SQL Server will work just fine in a Availability Group or FCI installation with a single domain controller. I have this set up in my lab and it works without issue. In this situation, you will be fine.
Generally, the design diagrams you see are splitting things between sites. So you have a DC and a SQL node in each site. If Site 1 fails, you can fail over to Site 2 and still have authentication still work. If you have only one DC, and the site where the DC is offline, then authentication will fail causing issues for your users.