waterway=derelict_canal allows a distinction between mapping a length of canal which is now dry or filled in but can be recognised by clues on the ground (e.g. gaps between the building, embankments, towpath, bridges, lock walls, or dry depression in the ground) and a channel still containing some water but no longer regularly used for navigation by the intended vessels: waterway=canal + disused=yes. The former may not be recognised as a derelict canal, whereas the latter is obviously some kind of water channel. This is particularly useful for creating maps for canal restoration projects where an organisation is working to restore the canal to navigation.
A further tag I have used (probably uniquely) is disused=no:isolated for lengths of canal that have been either been maintained to support, or restored to, navigation but cannot be accessed from other normally navigated sections because of blockages, such as lowered bridges or other derelict or disused sections. Sometimes these sections support a trip boat. Most renderers will ignore this tag and just render the waterway=canal tag.
request to improve documentation
To people using this tag: please, update and clarify documentation of this tag on OSM wiki. For example it is even unclear whatever it should be expected that it can be applied to canals no longer filled with water.
"used for transportation, waterpower, or irrigation" so filled by water seems intended as implied - but canal that is drained and converted into cycleway also fits "transportation".
On the other hand name (at least for me) implies canal where earthwork is present but is no longer filled with water.