However, it is quite badly out of date. Quite a number of the MCUs listed there have been successfully decapped by Caps0ff (including all of the Taito C-Chips) or independently acquired and dumped by others (mostly Team Europe). Also, a considerable number of ICs listed are now suspected or confirmed to be non-MCUs or have no internal microcode.
A list of all undumped MCUs in MAME would of course be much longer than this one.