Compiling sway with libinput < 1.1.0 results in the error: /var/tmp/portage/dev-libs/sway-0.8/work/sway-0.8/sway/commands.c: In function ‘input_cmd_accel_profile’: /var/tmp/portage/dev-libs/sway-0.8/work/sway-0.8/sway/commands.c:1283:31: error: ‘LIBINPUT_CONFIG_ACCEL_PROFILE_ADAPTIVE’ undeclared (first use in this function) new_config->accel_profile = LIBINPUT_CONFIG_ACCEL_PROFILE_ADAPTIVE; ^ /var/tmp/portage/dev-libs/sway-0.8/work/sway-0.8/sway/commands.c:1283:31: note: each undeclared identifier is reported only once for each function it appears in /var/tmp/portage/dev-libs/sway-0.8/work/sway-0.8/sway/commands.c:1285:31: error: ‘LIBINPUT_CONFIG_ACCEL_PROFILE_FLAT’ undeclared (first use in this function) new_config->accel_profile = LIBINPUT_CONFIG_ACCEL_PROFILE_FLAT; ^ Accel profiles were added in libinput-1.1.0.
Last <dev-libs/libinput-1.1.0 version was removed from tree several days ago. dev-libs/sway-0.8 entered the tree only after this happened.
There is currently no version of libinput in gentoo that is too old to build sway. The oldest version in portage is 1.1.0, which works correctly. Therefore, I believe it is not necessary to specify a minimum version for the dependency.