Are you perhaps thinking of when Atlassian removes the Mercurial repositories hosted on Bitbucket?
Mercurial isn't going anywhere or shutting down anything; it's open-source software with some very large users and committed developers.
(The majority of engineers at Facebook all work in one enormous Mercurial repo. Facebook naturally employs a few people to work full-time on Mercurial, and I don't think they're the only ones.)
Atlassian's decision about Bitbucket is regrettable. Especially regrettable is to actually delete repos for so many open-source projects that may not have active maintainers (rather than keep them online but read-only.) To my mind it marks a stain on their reputation that should make anyone think twice about relying on Atlassian for years to come.
But you should be able to avoid losing the source to any of your own dependencies. Between now and May 2020, go through all your dependencies and make sure you make your own clone of all the source repos.
The original programmer is long gone, and I while I am a semi-competent Python scripter and a slightly above average C programmer, I'm not looking forward to having to dig deep into this code, and patch it up to keep running.
In my very brief experience with python over the last few weeks this is very common. Half of our dependencies were abandoned before python 3 existed, when mercurial shuts off the hg we'll even lose the source to some of them. Python 3 get's the blame but the real problem is that the company has ignored maintenance for decades, it's entirely there own fault. There's no "business value" in maintenance until the whole lot needs to be rewritten.
Another underlying cause is package tools like pip, they make it too easy to take on dependencies with zero thought given to their maintenance windows and management think the can just ignore upgrading/replacing them regularly.