I had warcraft 3 installed on my OS disk(yea my mistake) and auto updater tried to push 20GB+ update on a 4GB game that was on disk with ~5GB space left.
Was able to confirm uninstall button present in Client version 1.19.1.11809 on Win 10 Pro v1909.
Cannot say if auto update is meant to check for disk space during patch process.
Yes you are right, there is a way to uninstall from the client.
Took me 20 minutes to find where it is x[. Interesting UX design, but not a bug.