Although on this forum we have the habit of calling it MESS, officially it doesn't exist anymore. 2015, the MESS project was absorbed by MAME https://www.mamedev.org/releases/whatsnew_0162.txt ; the MESS developers became part of the MAME team. And over time, remaining traces of the separation between MAME (arcade targets) and MESS (everything else) were removed.

There i̶s̶ was still a MESSUI fork over here: https://messui.1emulation.com/
CB-Emu was a fork of that.

All the current chess computer emulation development is in MAME. Even though CB-Emu is finalized, new chess computer drivers wil continue to be added to MAME.