Bitmessage is a P2P communications protocol used to send encrypted messages to another person or to many subscribers. It is decentralized and trustless, meaning that you need-not inherently trust any entities like root certificate authorities. It uses strong authentication which means that the sender of a message cannot be spoofed, and it aims to hide "non-content" data, like the sender and receiver of messages, from passive eavesdroppers like those running warrantless wiretapping programs. If Bitmessage is completely new to you, you may wish to start by reading the whitepaper https://bitmessage.org/bitmessage.pdf

Not sure how up to date it is, version 0.6.1 is dated 2016-08-21, the latest one  0.6.3.2 was released 2018-02-14
https://github.com/Bitmessage/PyBitmessage/releases/

According to https://www.reddit.com/r/bitmessage/comments/cjxk9s/current_status_of_pybitmessage/ (2019-07-30) development is on-going.
This can also be seen on github, e.g. https://github.com/Bitmessage/PyBitmessage/issues/1712#issuecomment-830636842
One of the difficulties seems to be the transition from Python2 to Python3.

Uses proof-of-work.

Found on https://karl-voit.at/2014/04/10/bitmessage/