It's funny, when TDIs first appeared here in 1996, they were manual only. It wasn't until 1998 when the New Beetle came out that you could get a TDI with an automatic, but as the years progressed, manuals' numbers went. When the B5 Passat [finally] got here with a TDI, they were automatic only. And all the later V6 models sold here are also automatic only, as was the Audi A3 TDI.
Then Dieselgate happened, and most of the rare manual NMS Passat TDIs were removed from the road!
So yeah, I'd say that car is pretty rare, but especially rare given that fewer people who drive manuals are willing to part with them in the first place.
For a 2015 Jetta sedan, I'd guess it was probably 10-15% were manuals. And they had a less-optioned TDI available that year, too (with steel wheels, no sunroof, and a lesser radio).
So kudos to you for having one! Enjoy it! After 2024, there will be no more manual Jettas sold here AT ALL. Let alone the fact that yours is the last diesel!