Not all developers publish their addons to our website, including some people with commercial addons they sell independently, so we still wouldn't reach everyone if we did that.
We made this clear in the release notes, in this forum posts, and by actually implementing the change in a beta release, so if it was broken presumably users would have been reporting that to you. This is the same way we approached the changes to the minifier last summer. Besides there seem to be lots of addon developers who aren't actively maintaining their addons, so I would think even if we reached out to everyone we could find (which as noted would still be an incomplete list), a bunch of developers would still not act on it until it's too late. We also can't be sure whose addons will actually keep working fine, and whose will be broken, and also it's not clear if most addons are actually fine and there's no significant problem, or if there's going to need to be wider changes. In this case since if you'd used strict mode - good practice since 2011, and already built-in to the addon SDK downloads - there was no problem, which seemed to be a signal it was probably a minor issue.
I appreciate maintenance issues like this can be frustrating, but many platforms require regular maintenance (especially mobile platforms, as iOS and Android often have annual changes to their requirements), and it's impossible for us to force addon developers to check or fix their addons no matter what we do. I think there is some level of responsibility for addon developers to keep an eye on developer news, release notes, listen to feedback coming from users, and maybe do a quick test every couple of months after a new stable release comes out. It's something we have to do with Construct itself, and even then we occasionally miss things, which can be a pain but then we just take action and fix it as soon as we can.