It does work but now I can't find how to update the maps in BaseCamp. I keep getting a 'Map out of date' and when you click on 'update map' it just takes me to a useless page on Garmin's website.
The “map out of date” message is simply a feature of Basecamp that triggers when the software detects that some of the installed maps are older than a year (I believe). Nothing to do with macOS Sequoia.
As SUSSAMB mentions you need to use Garmin Express to update maps, Basecamp does not have this functionality.
