I'm not sure exactly when it was added, but Defold now supports tile rotation, and .tilemap files created by the current version of the engine have rotate90 as a cell property. But when I export a map from Tiled to .tilemap, it doesn't carry over rotation flags.
Apparently rotate90 was added about 5 months ago and released with Defold 1.2.191 beta. I’ve now added support for this in the Defold export plugins, which will be available in Tiled 1.8.6 (alternatively you can soon download development builds from here if you’re logged in to GitHub). Thanks for notifying about this feature!