I know an individual provider who used to have a robust history, but recently she has become less reliable and is flaking on appointments.  Is posting a review about a "no show" an appropriate review, or is the review function only for actual meetings?

As far as I know, you can't post a review of a no-show.  You can post about it on your local board and tell what happened to you.

Swim

With perhaps the exception of a rip-off review if a depost was paid.

 
As swim notes, this is the type of thing to discussion boards, local or the one where the sessions was supposed to have occurred, if you were traveling. If perhaps you've had multiple cases the general board might be good -- or if it was about a touring provider the general board would also be appropriate.

Much appreciated

It's what this board is for. Glad to help.

 
You didn't ask but if you're interested in seeing what has been said before you can search on NCNS, no shows or ghosting/ghosted and probably find plenty of posts and related discussions.