They are still used but not as much, some TO-3's are housed internally.
Crown Macrotech 5002VZ
The Macrotech is probably about a 25 year old design though, could even be older than that. That was originally the MA5000VZ, the largest standard amp in that series. That line was discontinued about ten years ago or maybe a few years more. Its replacement was the Crown Itech line, which was one of the first big production amps with a DSP front end on it.
There was also a 3-phase amplifier in limited production (a 3-phase power connector!), the MA10000, which was installed as a distributed amplifier in large places like baseball stadiums. Distributed systems have an impedance-matching transformer located at every speaker, and thus the wiring caries a high voltage and can technically afford to be cheap and very long runs. You see distributed systems all the time in large public places like office buildings and shopping areas too.
These were all designed and produced prior to Crown's audio division being acquired by Harmon International (an audio conglomerate founded by Sidney Harmon of Harmon-Kardon). Prior to that, Crown was also producing close cousins to that amp, shaker table power supplies, which are used in factories and assembly lines. To this day I think they are still producing such gear.
Anyhow I haven't seen those TO-3 packages in use as often in newer products lately. My guess is it is likely cheaper to use a larger amount of TO-220 packages instead. An EE who is more up to date would probably know better, but just consider that more parts increases failure rate. Especially if they are made to lower thermal tolerance standards (I would imagine a TO-3 can dissipate more power than a handful of TO-220s - though I wouldn't know that part). What I have heard though is how when one package fails in an array like that, it can take out other packages. Not that a TO-3 failure wouldn't do the same thing though.
Crap. Looks like another one bites the dust. Power One, a company which built nice open-frame power supplies with TO-3s, was recently bought out by someone else. I used to spec their supplies quite often on updates and rebuilds. Hopefully they don't hack off all the good parts.
Double crap. On November 14, 2016, Harman entered into an agreement to be acquired by South Korean company Samsung Electronics. On March 10, 2017, the company became a subsidiary.
Okay I think I'm just going to stop before I start to get depressed over this.