We also had finer-grained -nobezels, -nobackdrops, -nooverlays, -nocpanels, and -nomarquees switches, but those all went away too. It sounds like Vas is going to make it so you can tag things as arbitrary descriptions and then disable stuff with those tags, which is great, but in the meantime there's no great options to control what gets displayed.
I was wondering about this as well, if legacy artwork support is maintained for at least 1 year (it seems?) wouldn't it make sense to keep these switches during that period as well? What's the plan for the old artwork? Will it be automatically updated to the newer format or does it require manual conversion?
I liked these switches because they were very straight forward to use and seems there will be no way to do this in an easy way with the new format, so right now if I want to only see the overlays for a game and no bezels (thinking on Vectrex for example) what would be the best way to do it? Create different artwork folders for bezels and overlays and then only feed mame with the overlay folder through CLI?