Blizzard's Midnight changes disable combat addons in end‑game content while simultaneously rolling out in‑game alternatives, breaking add‑ons that depend on combat‑time data and the combat log. WeakAuras devs say there won't be a WA release for 12.0, so addons relying on COMBAT_LOG_EVENT_UNFILTERED, secure frame/templates or live unit/spell APIs will need redesign or must rely on Blizzard's new built‑in UI systems.
There's a discussion of "UI & UX Improvements" that features some info on addons.
DBM author describes the changes in detail (prior to the day 1 alpha update). Notably, they haven't commented on changing the CHAT_MSG restrictions since the video came out.
Blizzard offers reasonable changes to addon functions.
WeakAuras won't be released for Midnight, so players on that client will lose a widely used custom notification and HUD system. Because WeakAuras depends heavily on the Lua API, FrameXML UI systems, and dynamic event/animation hooks, its absence suggests Midnight's addon API/sandbox differs significantly and other complex addons may face similar porting hurdles.