In its current form this will affect about 80% of all addons. The scale of change is the equivalent of the difference between what mods could do in Vanilla compared with the restricted combat environment introduced in The Burning Crusade. The most impacted by this would appear to be GSE and WeakAuras.
As with any expansion it is a moving target. Mods are enabled and disabled on the alpha at random. They are currently enabled and I have a proof of concept able to replicate the core aim of GSE. Is the PoC able to create a macro - no. Macros in this new environment are limited to only what is in /macro and a max of one action and total of 255 characters in each. (Essentially the equivalent of one action block in GSE.) What I am able to do is create a button that directly casts spells and abilities when called (clicked) by a hardware event. Combining a static macro with this button will replicate the bulk of what GSE does.
What does this all mean? There is around 520 hours work remaining over the next few months to have a successor to GSE available for TWW. This essentially is a new mod from the ground up with as much change as existed between GSE 2 and GSE 3.
As @Lloskka was able to advise you, I have been following this closely and keeping GSE’s Patrons informed of progress. They are the people who have input in the shape of how this will change going forward. If you are not a GSE Patron and interested in following this I would direct you to monitor GSE’s GitHub as this is where all changes to GSE are advised and discussed - GitHub - TimothyLuke/GSE-Advanced-Macro-Compiler: GSE is an alternative advanced macro editor and engine for World of Warcraft. It uses Travis for UnitTests, Coveralls to report on test coverage and the Curse packager to build and publish GSE..