I don't think it's been a thing for about 18 years, meaning it was only a thing for about 2 years of MAME's life.. While it was hand maintained in the very early days, it was later just generated based on the flags in MAME so didn't actually provide any unique information anyway.
MAME still has flags for working / not working machines, the internal UI can filter by them, although an external frontend like QMC2 can do a better job.
Note, it's not a games list these days, it's a machine list, there's a lot more than just games supported, again a good external frontend will be able to sort by given criteria such as if the machine has coin slots etc. tho.