Signals (v1) is now removed. Its removal was announced in 1.68 and its deprecation was announced in 1.54. Boost 1.68 is the last release that provides this library. Users are encouraged to use Signals2 instead. The Boost community thanks Douglas Gregor for his work on Signals which served its users well and which also inspired Signals2.
From https://www.boost.org/users/history/version_1_69_0.html: