Thanks for raising this concern. This particular example is not a problem in practice because React will not fire the warning about setting state in the short period between unmounting and cleanup. (We have special code and tests specifically for this case.)

So code like this can stay written exactly as it does today. As noted in the blog post we’ve only had to modify ~20 components out of thousands so although this change may cause some breakage (please report anything unusual to the issue tracker!) we’re fairly confident that such common patterns continue to work.

(Edit: added this to the post.)