We don't have an email list for all addon developers. We don't actually know who they are - addon developers work entirely independently to us.
Let me help you with the list of addon developers. Here's a full list of C3 addon developers user id's that have their addons published in the Addons&Extensions section. You just need to retrieve their emails that they, inevitably, have, otherwise they wouldn't be able to publish an addon on construct.net.
I believe these people deserved a tiny bit of your extra effort of targeted informing that there was a new feature that could potentially break their addons. Since it's not their job to read every release alert that you send out in plenty.
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198994
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Next time you need someone do the most basic work, don't hesitate to ask for my help.