[boost] [archive] Reason for modified BOOST_ARCHIVE_VERSION in 1.56 ?

Hi, the commit for the modification of BOOST_ARCHIVE_VERSION from 10 to 11 in modular-boost repo is not documented, except that it is due to release. What is the reason for 1.55 and 1.56 archive not being compatible (nothing in release notes). Greetz, Ingo.