Yup, that and the Slow Races (forward and backward!).
Skills don’t get obsolete, though I suppose they can be said to go “out of fashion”. But Horizontal Idling is kind of not a stand-alone “trick”, I consider it more of a utility skill, most useful in uni-sports as I mentioned above. Regular idling is more relaxed, easier, and more visual if you are performing for an audience. Horizontal Idle is not impressive for an audience to watch. Like Coasting, which is 100x harder, the audience mostly doesn’t realize you’re doing something hard, beyond the actual balancing of the unicycle!
Can be? Well I guess. If it’s only one person, that might be worth watching for a really long stillstand. Or a contest, with multiple people all going at the same time. But in competition format, where the equipment generally allows only one person at a time, that’s seriously dull!
I’m not sure why, but usually you only see balance poles like that when people are on skinny things, generally tighropes, slacklines, etc. Maybe because you need space for the pole to tilt to the sides, without hitting the ground. Because that would be cheating!
The photo linked above is of an informal Stillstand competition that was held at Unicon 11 in Tokyo (2004) Two brothers from the Philippines, who were both really good stillstanders, were the only ones left at that point, and continued to battle it out for about 15 minutes! That event followed an even more informal demo that the two brothers did at the Trials course on an earlier day.