/logout* invalidates the access token but also deletes the associated device
/delete_devices deletes the devices but also invalidates their access tokens
So essentially the same operation is approached from two different angles: one from the perspective of deleting a device (and invalidating all of its access tokens) and the other from the perspective of invalidating an access token (and deleting its device and other access tokens).
This makes /delete_devices a weird odd beast. It would make more sense to have a /logout/some instead.
Suggestion
The difference between "deleting a device" (via https://spec.matrix.org/v1.11/client-server-api/#post_matrixclientv3delete_devices) and "logging out an access token" (via e.g. https://spec.matrix.org/v1.11/client-server-api/#post_matrixclientv3logout) is not terribly clear. After some discussion in #matrix-spec, it appears there is no difference, since:
/logout*invalidates the access token but also deletes the associated device/delete_devicesdeletes the devices but also invalidates their access tokensSo essentially the same operation is approached from two different angles: one from the perspective of deleting a device (and invalidating all of its access tokens) and the other from the perspective of invalidating an access token (and deleting its device and other access tokens).
This makes
/delete_devicesa weird odd beast. It would make more sense to have a/logout/someinstead.