According to Wikipedia:
TDRS-M, to be renamed TDRS-13 upon entry into service, is an American communications satellite operated by NASA as part of the Tracking and Data Relay Satellite System. The thirteenth Tracking and Data Relay Satellite, it is the third and final third-generation spacecraft to be launched, following the 2014 launch of TDRS-L. (emphasis added)
In this answer I've suggested that this may have come to pass, as it is named
TDRS-13 in the TLE I downloaded today from Celestrak.
However, it does not show up in thier Tracking and Data Relay Satellite System (TDRSS) option on Celestrak's NORAD Two-Line Element Sets Current Data TLE page, on which only the following spacecraft which have TDRS in their name are found:
TDRS 3, TDRS 5, TDRS 6, TDRS 7, TDRS 8, TDRS 9, TDRS 10, TDRS 11, TDRS 12
Has TDRS-13 officially entered into service?