Unfortunately, it's now only available on a few routes and planes:
https://www.flyertalk.com/forum/united-airlines-mileageplus/...
I found it very comforting to listen to it during an entire flight, because you could find out what the pilots of your flight were actually saying about turbulence and flight conditions, including their ongoing efforts to avoid turbulence purely for passenger comfort. You could also hear from other pilots who had recently flown through the same region, discussing how turbulent it was and how long it was likely to last. Among other things, this can reveal that the pilots are aware of the situation and don't consider it threatening.
Part of the scary thing about turbulence can be wondering "do the pilots realize it's so bumpy?" or "are they concerned about this?" or "what are they doing about it?". Most U.S. flights don't really make a lot of passenger announcements about these questions, but if you listen to the ATC communications, you can get real answers (usually "yes", "not at all", and "trying to get a clearance for a different altitude").
In many regions there's volunteer ground-based coverage for live ATC streams:
It's obviously more complicated to identify the particular ATC region that your flight is in (and then manually change to a different stream every time you're handed off to a different controller!), but if you buy in-flight Internet access, you might be able to listen to some of these communications from the air even if you're not on a United flight with channel 9.
The biggest challenge is that ARTCCs (the high-altitude ATC facilities) have subdivided their airspace into sectors with different frequencies.
For instance if you look at
https://www.liveatc.net/feedindex.php?type=us-artcc
you can see the Denver ARTCC (ZDV) is subdivided into more than a dozen different sectors (and there's no guarantee that this list is even complete). So if you wanted to listen to these communications this way, you'd have to take a rather active role in selecting streams and accept that your flight would probably pass through regions with no streaming coverage.
(Edit: for example you might hear "(your flight number), contact Denver center one two eight point three two five, good day" in which case you would want to switch over to the "Denver Center (Sector 14 High): 128.325" stream. If you succeed in switching in time, you should then hear your flight call "Denver, (your flight number) with you at (altitude)". But if you missed this instruction, you might have to spend an inordinately long time listening to random streams in the hope of hearing your flight's callsign, or do significant research on FlightAware to figure out what sector you might be in. Quite a chore compared to United's service...)