Back in 1.13, if you had an armor stand that had the visible tag set to false (or is it the invisible tag set to true?), the armor stand would be completely invisible, but if you then went into spectator mode, you would be able to see it transparently. This was useful to see where all of your armor stand markers were. Im sure this applies to other entities too, where being in spectator mode made it easier to figure out everything going on that you couldn't see. If this is no longer going to be a thing in spectator, then there should be a simple way to make invisible entities, mainly armor stands, visible without having to run a particle command on all of them or some other complex way to add visibility.