My current game I DM the PCs are 5th level. It has been weekly (mostly, we are scheduled for weekly but a bunch of cancellations) online games for about a year now. I am running a 5e conversion of the pathfinder Iron Gods adventure path so the levelling has been milestone based along the AP suggestions. It has been a different feel and pace running shorter (3 hour) games than I did in person.
I included all my 5e campaigns where we played longer than a few weeks. Most of them are retired, but we played them for a while. Two were 20th level, one 13th, one 8th, and the present campaign is 6th.
Well no, this survey isn't meant to be an actual study. It's for entertainment purposes only.
The only "evidence" it has: it is evident that out of 78 voters, 19 of them have characters above 15th level. About 1/4th. Is that a small enough fraction for it to be considered "rare"? I don't think so. I mean, I don't think of Fridays as rare occurrences, and only 1 out of every 7 days is a Friday.