build2 packages for Boost libraries
The meanings of the Boost version components differ so much from Semantic Versioning that they are of no use for dependency management.
According to the Boost FAQ, the patch level is increased for maintenance releases, the minor level when new libraries are added, and the major level in case of "massive changes" such as "the reorganization of many libraries". The FAQ further says that backwards compatibility between releases cannot be relied upon (without qualification and thus, by implication, this includes maintenance/patch releases) and further recommends that users pin their Boost dependencies to specific release versions (again without qualification).
They have also made binary-incompatible changes in maintenance (patch-level) releases (see the diffs between 1.65.0 and 1.65.1, for example; the libraries with notable changes include Context, Fiber, and Thread).
With all of this taken into consideration (specifically, the absence of any guarantees about what can change between patch releases), one can only assume that anything, including public interfaces, can change between two versions, regardless of which version component was incremented.
For this reason we use the Boost version as-is. (This is also what Debian and vcpkg do.)