is nexus 6 still a good purchase? is nexus 6 at its end of life?

Quoted from the link in @Firelord 's comment above:

Nexus devices get Android version updates for at least 2 years from when the device became available on the Google Store.

Be it the release date or the Store date, N6 still hasn't reached 2 years yet, so rest assured it will receive N, though not with priority, since Google takes care of their latest devices first.

Also, although Google makes the promise stating 2 years, that usually expands longer. Nexus 4 received 5.1 and Nexus 5 received 6.0 officially, making them ~3 years, and both keep getting security patches up till today. Nexus 7 (2013) got N Developer Preview which also counts to ~3 years.

Finally, there's always custom ROMs to get you covered. Not guaranteed to work 100% perfect as Google's stock, but as time goes and bugs get squashed, they approach 90%~99% if you ask me.