So the latest version I show that we support is 6.20.531.5 which was made available March of last year. I don't see any other updates on the road map as of not so it is pretty unlikely that there will be anything more than the 6.20.531.5.
Thanks. I guess I will take up the issue with YouTube TV, whose app is incompatible with my HTC One M8 on 6.0.
I own another HTC One M8 with a custom 6.0.1 ROM and the app works there (once I manually install the .apk.)