If such tooltips are added, please enable them on hover too. This would work with stylus (and mouse), of course, but also e.g. Samsung phones have a built-in option to enable hover functionality with fingers.
I've seen it in a calendar app long time ago (to display event details in a month view), but it's probably not very common.
Fair enough, don't think I've ever even tried to press and hold in that context before, to me it has always been a metaphor for a context menu à la right click (the F-Droid one seems particularly useless seeing as it already has the labels?).
I tried it on a few more Google apps, Gmail, Drive etc. and is part of Material Design so I have no issues for the Android implementation of it personally. - Material Design