Thank you for mentioning this. Unfortunately, yes, ToroDB is no longer being developed. I still believe it's a fantastic idea, and provides significant value. But when it was being built, 5 years ago, the NoSQL (as in "abandon SQL") state of mind was too strong, and the value proposition was not well understood.
I moved to work on what's always been my passion and preference: Postgres, Postgres, Postgres. For those interested, StackGres[1] is what's now my company's focus.
Things may be different today with ToroDB. The technical foundations and ideas are still there. If there would be significant interest by entities that would like to contribute to its development, it could be considered.
I wish good luck to FerretDB. The task ahead is not easy: MongoDB protocol is very simple, but the API is terribly complex and full of nuances. Getting up and running a simple PoC is very simple. Getting from there to a production quality state with notable compatibility is very hard.
[1]: https://stackgres.io