Spotify doesn't seem to allow track numbers to skip like most other major digital music stores / streaming services do.  This results in tracks like https://api.spotify.com/v1/tracks/3d1q12DdseNeGhswp326WM which is listed on a release available in 183 markets, but this specific track is available in 0 markets - it seems to only exist on Spotify so that the rest of the track numbers match up to what's displayed on Apple Music, etc. (the UPC/barcode is shared between platforms).  I think it should be explicitly stated in all relevant Style guidelines that tracks that don't exist on that Medium shouldn't be added to a MusicBrainz release that has an "available" status (Official, Bootleg, etc.) In other words, all tracks on a medium submitted to a MusicBrainz release that is supposed to be available should have actually existed and been available on the associated release date.

Adding nonexistant tracks to a release can break the premise of parts of MusicBrainz (for example, the way Original Recording Date is calculated).

Additionally, the guidance for digital releases that change over time has been to take snapshots of the release (ideally, using the date of the changed content, not the date that you are taking the snapshot).  (In the example given above, all snapshots (to this point in time) that anyone would have taken would be identical, as the track has always been unavailable.)