Chat inside instances will be no longer accessible to addons. Lots of RPers use a chat addon and some of them will RP through a Dungeon or Delve or even Raid.
Blizz has clearly said that in combat instances chat addons will be broken. They will function outside of instances and the “Housing Instance” seems to be an explicit exception where chat addons will still work.
"…there’s a complete lockdown on the ability for addons to parse chat and addon-comms channels while in instances. "