With latest sway and seatd upgrades in edge, the user must be in the "seat" group, or else sway will not start:
[ERROR] [wlr] [libseat] [libseat/backend/seatd.c:82] Could not connect to socket /run/seatd.sock: Permission denied [ERROR] [wlr] [libseat] [libseat/backend/logind.c:653] Could not get primary session for user: No data available [ERROR] [wlr] [libseat] [libseat/libseat.c:79] No backend was able to open a seat [ERROR] [wlr] [backend/session/session.c:84] Unable to create seat: Function not implemented [ERROR] [wlr] [backend/session/session.c:218] Failed to load session backend [ERROR] [wlr] [backend/backend.c:353] Failed to start a DRM session [ERROR] [sway/server.c:53] Unable to create backend
With pma!2631, it will be fixed for new installations. Existing installations need to add the user to the group manually.
$ sudo usermod -G seat -a $USER