For find_nearest_contour that won't really work, as you'd also need to undeprecate the collections property, which is accessed by its implementation (as detailed in #27070 (comment)), and then you'd need to decide whether accessing that property just permanently triggers the conversion to the old-style internal representation or not (currently it does as that's the point of the backcompat layer). If it does then that means that any ContourSet on which one calls find_nearest_contour will revert back to a state where #6139, #11464, #23080 are broken. Perhaps we should keep that discussion at #27070 though.

I can't say I like keeping allsegs and allkinds around forever, but I don't really mind either.

@anntzer apologies - I didn't see your note on #27070. Lets move this to draft and discuss that first...

I have pulled this change into #27088.