It was from a simpler time ( reddit post / archived blog post) … “reddit is vast. Unfathomably, mindbogglingly vast. There were over 71 million unique visitors last month. Yesterday, there were 5,487 active subreddits.”
On old reddit, if you had the left sidebar enabled, they showed up there. I’ve got quite a few private ones that I set up there that group my interests. old.reddit.com/r/multihub/
On new reddit, with the left sidebar showing, there’s “custom feeds” where you can do similar things, however its discoverability is much reduced. It’s only really visible if you favorite it so it shows up near the top of some lists / drop downs. If you still have a reddit account, and create a custom feed, you can create one, adds subs to it, make it public.
However, the difficulty with this is two part… discoverability (again) and the single ownership. The French subreddit that is a directory of other French subs is shared ownership. You’ll note with the multireddit its one person who maintains it… and that leads to abandonment.
A way to solve that (on reddit… yea, I know, talking about the other place here) is to have the shared ownership of the directory subreddit then drive scripts that update a bot account that publishes the public multireddit. You can see similar efforts with shared ownership and scripts hub.docker.com/u/redditopenttd (it doesn’t update a multi, but there’s no reason why it couldn’t).
Similar problems would need to be solved on Lemmy (including discovery and ownership) for the “how would you do an aggregation of multiple /c/ across multiple instances into one “bundle” and how / who maintains or curates it?”