Do you still use words/phrases like groovy, bummer, far out, keep on truckin', gnarly etc?

Yep. 

As a linguist, I imagine you find some of the generational differences interesting. I do.

Groovy sounds wimpy these days, though I like it when I hear it.

No, I don't use any of those words.
In fact, I've gone the other way and avoid using the latest fad words.

Although it's impossible, I prefer writing to have a timeless quality. For instance, one can read Joseph Conrad's Lord Jim, and it doesn't take an urban dictionary to understand it.

You'd be surprised how often, on this site, I have to look up the meanings of American slang.

 

  iknowrite?

I can't even pronounce some Aussie slang.

A cunnilinguist? Seriously, when I was in college, the student humor magazine, "The Lunatic," put out a spoof football program for the homecoming game one year. One of the players was called Buster Hymen, listed as a double major in cunnilinguistics and nasal hygiene. This post was edited by Stu Spelling Bee at July 9, 2022 8:38 PM MDT

Far out.

A classic song:

I ain't no whippersnapper.

Eh?  Eh?  Speak up!