The purpose of #28207 was not to make the parameter undocumented but to remove it altogether; so nothing looks accidental there. In that pull request, asyncio-eventloop.rst had the following explanation why reuse_address was first documented as deprecated, then turned into a stub that just throws exception, and finally was removed:

When multiple processes with differing UIDs assign sockets to an identical UDP socket address with SO_REUSEADDR, incoming packets can become randomly distributed among the sockets.

In other words, the whole feature allowed to tap into a socket of another server and intercept random packets sent to it from its clients.

Right, I understand that. But it was only deprecated/documented as deprecated for create_datagram_endpoint, that PR mistakenly also removed it for create_server where it was never deprecated.

Ah, right, my bad. I don't know why it came to be that "reuse_address" =="the whole idea of shared binding". The PR really directly says that:

Due to significant security concerns, the reuse_address parameter of →→asyncio.loop.create_datagram_endpoint←← [...]

Edit: your bugs.python.org entry that I, of course, did not read already explains a nature of the mistake. Twice my bad.

Thanks @jcrist !