@MartinSleziak My opinion is that it's a close call, but I wouldn't have picked mg.metric-geometry which I think of more as at an interface between analysis and geometry. It would have been okay to omit a geometry-related tag in this case. No harm done though.

Thanks for the response. I left it as it is now - of course, feel free to go ahead and choose a better tag.

BTW I was a bit surprised when I found out that also abstract-algebra and discrete-mathematics are deprecated.

In fact, all three tags I know of which are deprecated here - abstract-algebra, discrete-mathematics and geometry - are used on math.SE. (Of course, I know that the two sites are different in many aspects.)

My impression of mg.metric-geometry was that this is the tag which I would typically use as top-level tag for things falling under plane-geometry, euclidean-geometry, etc.