Our application is still maintained, so we can find workarounds or restructure the form to use an extra popup (to the detriment of usability). But i'm sure many applications won't be updated, and as a result of this change, data will be silently corrupted when they are used. The browser has knowingly broken compatibility with the web application.

This is a Torvalds "don't break userspace" moment.

Fair enough, that's a valid concern. But it wouldn't be solved by allowing pages to opt-out of autofill, since they'd have to be updated to use those as well.