I’m not aware of multi-level ones in Canada, but there might be. I’m just not familiar enough.
That proposal is a bit odd in that it was proposed in 2010 and voted in 2018. My recollection is that discussions about using the tag took place much earlier, and much of the history is poorly documented. I reviewed the chat logs (line 54400) and found discussion that occurred in October 2013 and referenced earlier talk-us@ posts, so my guess is any discussion happened in early 2012 when usage started. I was unable to find the list posts. There was additional discussion in July 2014 (line 114534).
One community member was of the view that reserves should be cut out of the admin_level=4 states, and if you were in a reserve you weren’t in a state. Myself and others were of the view that reservation boundaries were orthogonal to any admin boundaries as they can cover multiple states, multiple countries, or lie entire within a city. I am still of that view.
A different type of boundary=* was considered best, and we were using boundary=aboriginal_lands
at that time, and that was considered better than trying to shoe-horn it into boundary=administrative and admin_level.
At the time, there were not many mappers who were mapping these, and there was some questioning if they should even be mapped. But then again, some prominent US community members were questioning if any admin boundaries should be mapped either.
Going farther back in history, in 2010 the rendering was mis-named Mapnik and labeled Nature Reserves with little NRs and Indian Reserves with IR. This was both racist and bad cartography.
The original aboriginal_lands proposal was made by a user who shortly after left OSM. I believe use started after he left. Around that time the community settled on aboriginal_lands as the best name under British English, given its use in Australia and Canada which are closer to British English than American. Indian is obviously out, as not being a preferred term anywhere, and being completely incorrect in Australia. Maybe in retrospect indigenous should have been used, but there are issues with any term. In any case, this is far from the first tag that isn’t named ideally.
What does this mean for sublevels today, over a decade later? We never thought about sub-levels at the time, although we did decide to include all indigenous reserves in the same tag, even though some are much bigger and operate differently.
I would avoid doing boundary=aboriginal_lands
and admin_level
. Come up with a more appropriate tag name, as there’s no usage right now.