I think that message was to let users know it's basically on it's last legs and could 100% stop at anytime. When the API changes started, most all of the instances ran into rate limits due to the number of requests. But it works just fine until that rate is hit.
Like I mentioned, I've been running my own private instance and I've never hit the rate limit so it's been working fine for me.
And I don't know about any API key. AFAIK, Libreddit uses the publicly available JSON, no need for a key (hence why it's read-only). Since it's still working for me, those JSON endpoints must still be available, just highly rate limited now
