What the mode is
Honour Mode and Custom Mode shipped in Patch 5 on 30 Nov 2023. Honour Mode is above Tactician. It adds more than 30 boss tweaks and a Legendary Action system, and it disables save scumming. Some exploits are removed only on this mode.
On a wipe, the game shows how long and how far the run survived. You can continue, which disables Honour Mode, or you can stop. The Golden D20 is awarded for completing the game with Honour Mode still enabled.
Hotfix 16 says the Quickload button is hidden on Xbox while you are in Honour Mode or in Custom Mode with Single Save enabled. That is Larian's name for the single-save switch.
Fight assumptions that break
On Honour Mode, extra actions from Haste and similar effects do not trigger Extra Attack. Action Surge still does. Pact of the Blade's extra attack does not stack with a martial Extra Attack on this difficulty.
A legendary action is spent once a round and then returns at the start of the next round. Hard control that leaves the boss fleeing, incapacitated, stunned, unconscious, or restrained stops it from firing.
Custom Mode can load the Honour ruleset on a new game as of the 19 Sep 2024 note from the official account. That is a place to learn a boss without spending the Honour save.
A body change you cannot reload
The illithid page's third gate is a body change. This mode has nothing to load if you dislike it. Practice the ruleset on Custom first if you want a rewind.
Official site: baldursgate3.game.