thank you for looking! Looks as if that message is indeed dead. Without a translation map, it would be fairly useless...
The original use I had for it was in a Morph plugin (goes back to R8) so the various morph shapes could keep track of changes done to the original mesh. It's not properly working there any more; guess I know now why 😉
My current use is a Symmetry plugin I just finished. Right now, it only works by mapping right points and left points, and duplicating positional changes. I am now looking into possibilities to track added or removed points (and maybe polygons) on one side to duplicate that too. The TRANSLATE messages used to do that, including special information like "point A was merged with point B", but naturally all tools need to support that, and apparently the new kernel has moved on to some other method.
I guess I'll investigate whether MSG_POINTS_CHANGED is now carrying the required information...