So far apart from the solution that is not a solution but a breaking, if the strategy is lock the door with - login_required (setting), then in that scenario, to mitigate the negative traffic hit effects, if you rely on search traffic, is to have something to see but not everything.
WP frontend / Discourse login_required site
(more work, more hosting costs, support etc.)
Things that would also help but aren’t built with exactly this problem in mind:
Published Pages if developed with a dedicated listing page, some options to configure, could act as a bridging landing page where users can see some public front content with a register to read more prompt
– allow published page listing on own page /pub (make home page)
– allow published pages listed on login_require page
– allow custom category or latest on login_required page
I only found Published Pages a couple of days ago as a feature while trying to find a solution to this problem, and iirc even before the AI conundrum previous users have requested similar listing feature for published pages.
A more configurable purposed treatment of published pages is to my mind more preferential than a whole WP frontend bolt on, if needing to resolve some connection point that is public facing.
List Topic First Post only
Show only the first post of any topic and require login to read comments. I’ve seen similar suggested at least once and given the thumbs down but in this context it require re-evaluating.
Also regard these suggestions as an incomplete list, merely potential band-aids for part of, and not all of the problem.
Meanwhile I’ll revert to terrorising this topic with loads of feelz 
How are we all feeling about ChatGPT and other LLMs and how they'll impact forums?