Steve was indeed planning to split off obsolete histograms into a separate file. 
  For the flags entries, we don't have any concrete plans currently.  Ideally, we'd keep an append-only mapping somewhere server-side, and drop the requirement to keep histograms.xml up-to-date with historical information.

FWIW, both git and gerrit seem to be fine in terms of diffing the files.  Which tools are you having trouble with?

Good to know it's already been considered, thanks!

I remember rietveld giving up when trying to do a diff between intermediate patch sets. And it also just takes ages to load on codesearch. Or doing a blame there, etc.

The other idea we've discussed is having the server-side infrastructure store history, so that things could just be deleted at TOT and the dashboards would still know the historical descriptions and which things are obsolete (by virtue of them having been deleted.)

If we do that, then we don't need separate files for obsolete entries, so on the Chromium side it will simplify things a lot.

Re #2: Thanks! Rietveld did indeed have an issue with this file, which Gerrit has mostly fixed.  Codesearch is indeed still slow :(

Re #3: True, though some histograms provide some useful info in their <obsolete> message, such as what histogram replaced them.  It's admittedly not too common, though.