I was thinking about the new patch and how it breaks mods like decursive and auto heal cancel. I understand that the change to SpellStopCasting() makes mods unable to cancel heals without player input. However, what stops mods from targetting people that get debuffs on them and then having the player spam one button (input) to cast cleanse?

I'm not arguing the changes, and I think they're a step in the right direction in terms of mods completely taking over end-game raiding, just curious as to what makes it impossible.

When Blizz UI is reporting mod for "bad behavior", it isnt always correctly points to the addon that is doing it.
If you have an idea how addons work - they hook some standart function. When more than one addon hooks same function, this creates a hook chain. And Blizz UI is always blaming last addon in hook chain.
Plus its not only SpellStopSpellcasting is changed, hooking to movement keys and functions is not allowed to 1.10. Thus ppl that bounded decursive to movement keys will see this message.

I enjoy the "OMG I CAN'T BELIEVE BLIZZARD BROKE SO-AND-SO MOD!" threads on every new test server. :)

It would be pretty difficult to break decursive, since it uses pretty basic scripting functionality. They know they can't break it without breaking many, many other things, so they won't. They're just not designing Lucifron-type encounters anymore.

Yeah, all of the "blocked" messages on Test are a result of Blizz's disabling hooking to the movement functions. This breaks things like Autobuff, but doesn't disable much else. It will just be a pain for UI writers to update a lot of stuff, and for us to find new versions of everything, but that's about it.