What Blizzard has said
In a BlizzCon developer interview, Mike Nuthals said Forever will bring over in-combat addon restrictions that prevent the kind of programmatic calculations used to automate marking or communication for players.
Important: Blizzard has not yet published the full Forever API, a compatibility list, or an exact rulebook for Classic addons. The beta will be the first reliable place to test individual addons.
Likely safe assumption
UI customization and informational addons may remain possible, while computational combat automation is the area Blizzard explicitly wants to restrict. Exact compatibility with Classic Era and Retail addon versions is still unknown.
What beta needs to answer
Which Classic addons load unchanged
Forever interface/API version
Combat-log access
WeakAura limitations
Raid-marker automation
Communication automation