I looked into how these obsolete libraries are used by other Boost libraries, and they cannot be simply removed. It would be a non-trivial endeavor for related libraries to stop depending on, say, Boost.Move. Unless the library is no longer depended on, it has to stay in the release regardless of opinions for or against removal.
I agree,
Therefore the more important question becomes: what level of effort should be invested in removing the dependence on obsolete libraries from the non-obsolete Boost libraries which use them?
What do we gain? I added a new MSM backend. I could have reworked the old one instead. But MSM users do not like me breaking their code. And as I did accidently added a bug while fixing the new backend, it would have been a serious bug for 2 Boost releases. Forcing me to remove usage of MPL would force me to remove the old backend and hope for the best. Sorry, no. I will remove the rest of the MPL in new backends but disagree with getting all of MSM declared obsolete for trying to keep part of it backwards compatible.