This first implementation of signals has been deprecated since the 1.54 release. (When) are we going to remove it? It is under management of the CMT, and it has been deprecated for over 10 major releases. I would like to suggest we remove it in the Boost 1.68.0 release.
i had been working on one project, that was never migrated from signals to signals2, as attempts to migrate miserably failed: compile times significantly increased and the msvc linker would run out of memory. this project is currently migrated away from boost.signals (1 and 2) to a more lightweight solution (nod signals), but i can imagine that there are some projects out there which still use signals(1) and which would run into trouble when it will be removed completely. otoh i wonder if signals1 causes much maintenance overhead, that justifies potentially breaking downstream code