For example, I've seen this on a boost.heap issue:
boost.heap is in maintenance mode. but i'm more than happy to integrate PRs
What does maintenance mode mean? Does it mean it's deprecated?
Maintenance mode (wikipedia article):
In the world of software development, maintenance mode refers to a point in a computer program's life when it has reached all of its goals and is generally considered to be "complete" and bug-free. The term can also refer to the point in a software product's evolution when it is no longer competitive with other products or current with regard to the technology environment it operates within. In both cases, continued development is deemed unnecessary or ill-advised, but occasional bug fixes and security patches are still issued, hence the term maintenance mode. Maintenance mode often transitions to abandonware.
Sometimes, when a popular free software project undergoes a major overhaul, the pre-overhaul version is kept active and put into maintenance mode because it will still be widely used in production for the foreseeable future.
In the boost heap case most likely it means that the library is considered feature complete and bug free and the authors are not working on it anymore, but they will accept PRs from the community. It doesn't mean it's "deprecated"