That release, Multics MR12.5 (MR = "Multics Release"), was released to customers in November 1992. The last Multics system was shut down in 2000.
The software can be downloaded from a website at MIT, though it requires specialized hardware to run on, so don't expect to be able to run it. My name appears a few times throughout the software, as I worked as a Multics software developer from 1983 to 1988. The MIT site incorrectly states that Multics development was ended by Bull in 1985--that may have been the time when Bull decided to pull the plug, but there was still development (though primarily bug fixing) going on in 1988 when I left.
One of the pieces I wrote was a rewrite of the interactive message facility, in some ways a predecessor of instant messaging (except that it operated on a single timesharing host rather than over a network between hosts).
Most of the software is in the "ldd" hierarchy (for library directory directory, the directory of directories of libraries). The software is in Multics "archive" format which is similar to Unix tar files. The message facility software is in /ldd/sss/source/bound_msg_facility_.s.archive.
Kudos to Group Bull, the copyright holder of Multics, for making the software open source. Bull purchased Multics as part of its acquisition of Honeywell's Large Computer Products Division in the mid-eighties.