I don’t recall seeing people hand-pick manure from arenas very much until the high-tech – and very expensive – surfaces with a wax component became popular a decade ago. One of their strongest features is a lack of dust but also no need for watering. The surfaces come with a caveat that manure must be removed religiously, because it breaks down and becomes dust, and then that very expensive lovely footing is no longer so lovely.
The basic rule for these arenas is that, if your horse poops, you pick the manure as soon as you’re done riding. You don’t expect someone else to do it for you, and you don’t wait any longer than necessary because once other horses start tromping through it, then the little bits of manure will mix into the footing. A corollary to the rule is that if someone is on foot in the arena – trainer, spectator, anyone – they go pick manure immediately when it falls.
Now, I’ve started to notice that even arenas with more ordinary sand footing are being hand-picked following the same rules. Basically, the people who use that arena are all in this together. A clean arena helps to reduce flies and dust and also potential slick spots. This de-pooping procedure is particularly true with indoors and covered arenas and even occasionally with outdoor arenas, maybe not huge ones but certainly smaller dressage-sized rings.
It helps in an indoor to have a muck bucket and fork stationed in each corner or at least at each end of the ring. With covered arenas, the usual practice is to just toss it over the rail onto the grass outside.
In Europe, apparently, manure removal in arenas has been a practice for a lot longer than here, and they even have specialized tools, like a small rake combined with a container on a handle. I expect the tools will catch on here as well before very long.
Several years ago, the barn I was in moved the horses en masse to winter quarters with an indoor arena that happened to have the new high-tech waxed footing. I was already accustomed the de-pooping procedure, but a couple of the other boarders protested, saying it should be the job of a barn worker at the end of the day. No, it was explained, this is everyone’s responsibility. Besides, a barn worker can’t be posted at the ring just to pick manure all day there. Arena picking has become just like other good manners in a shared boarding barn, like always sweeping the grooming area and always passing left to left. A clean arena is just nicer for everyone.