Up until now using undocumented feature meant we need to update addons if you make changes to the engine. I.e. you add RTL support to the text render and the animate text addon breaks, to fix that the addon dev needs to bring the addon back in line with the changes of the text renderer. (btw I don't think you got many, if any, people reporting that to you, but instead it was reported to Skymen).
As you will always have some form of text rendering in the engine it's save to assume that while it might change, maybe even drastically, it is near impossible to be removed completely. This meant using undocumented features was a larger maintenance burden on the addon dev, with a small chance of being impossible to fix.
Depending on the undocumented feature we could make an educated guess how risky it is to use. But now what you are saying means that you are purposefully intending to make all of this impossible permanently and you want to do it as soon as possible. Now we cannot weight the risk, because now there is Damocles sword over every undocumented feature.
It's not clear to me what benefits this will bring, it sounds like a waste of time.
I also think you underestimate how much value 3rd party addons bring to your product and it's a bit disheartening to hear that instead of trying to give more power to those developers you want to take power away. I also think you might not be aware how many developers and games moved away from Construct after hitting some limitation and then made highly successful games on other engines, these could have been hit games made with Construct.
... I could go on but I guess all these points have already been brought up by others