Oh man, so I only just learned from looking at the "what's new in Chrome 86" release notes that they quietly completely changed the file system APIs starting in Chrome 86. This means r218 will not detect that the feature is available and so won't show the local file/folder save options.
This was a really nasty way to find out they've made breaking changes and really throws a spanner in the works! In the past they've at least given a few releases notice with deprecation warnings to give us time to upgrade... looks like they've just skipped that this time.
We're going to have to make a range of changes to Construct to update our code for all the changes they made to the API. I also would be reluctant to ship significant changes directly to the stable channel, since they won't have been thoroughly tested. I think the best thing to do here is to fix it for the next beta release and see how that goes before deciding if we backport anything to the stable release.
So in short I'm afraid this will further delay the feature being reactivated again.