What about a checkbox ‘do not show this warning until the next time the map pool changes’ if thats a concern?
One of the things i found very frustrating when starting out on the matchmaker was it saying a match was found yet it would stay stuck on the loading/waiting screen for ages until i gave up and rejoined the queue. i suspect this was due in some cases to people not having the maps.
It also wouldve helped me in the first 1-2 games i tried to join as i dont think i was aware i should download them in advance and suspect there are other new players who may not be aware. I found failing to get a match very frustrating and the issue happened far more as a newer player (made worse by the lack of feedback indicating what went wrong). Map pool changes will also likely cause issues since people may not realise there are new maps they need to download.
Another benefit of this is that it’d then significantly reduce the downside of larger map sizes (whether by enough to warrant such maps I’ll leave for someone else as I don’t have strong views either way)