While child support and health coverage go to 21, that may or may not be true for dental coverage provided by the company or state government. Dental is usually considered separate from health by insurance companies.. For why SS15 is covered and SD20  isn't, DH is going to have to ask BM. But, note that my dental insurance says: Dependent children are covered under these benefits up until the end of the year that they turn 19.

If she getting retirement benefits she might be only able to add those whom she can claim as a dependent on taxes. That's the way it works for us.  

I'm in NY too, so I'd be shocked if the state didn't require dental coverage to go to at least 21, especially for state workers, even after retirement.  My SS is 21 and still covered in all ways by BM's insurance. She's not a state worker, but state worker benefits are even better. 
 

Perhaps SD moved out and BM doesn't want DH to know because CS would end? Can he just ask BM why she no longer has insurance?

ETA: do a Google search, my guess is that info on state worker benefits is all online. BM here works at a private college and hers were all online. We were able to use that info - we found she was eligible for a small scholarship for SS that she hadn't mentioned using. She was pissed because she had to share that benefit with DH. 

Make sure any payments DH makes go directly to the provider and make sure they are for the child

Before she lost custody our BM sent DH an email demanding $100 for the full cost of taking SD to a dental cleaning.  The problem is, cleanings are 100% included on our insurance, AND BM is responsible for half of everything.

So DH responded that he would take SD to the appointment.  So then BM responded that it might not be $100 after all.

Turns out she wanted the money for herself just because, or wanted DH to pay for her cleaning.  We never heard a peep from her after that, she backpedaled and said she'd take her.

She's 100% grifter, all the time.

DH should tell BM to F-off.