MAME is about the emulation and preservation of basically everything electronic. Arcades, computers, consoles, calculators, LED Tiger games, you name it. If it has a CPU (sometimes, even if it doesn't have a CPU), it's fair game for MAME.
There are no hard cut-offs, but contemporary systems are rarely implemented. MAME's focus on accuracy to a low-level degree means that most modern systems would be painfully slow to emulate.
Contemporary arcade games are usually just a PC in a box with external arcade specific hardware making up everything else. Probably better to hack the game into thinking it's running on the right hardware than the other way around at this point.
Case in point: I was at an arcade the other day where they had a Beat Saber VR setup. At one point they reset the machine, which displayed a Dell UEFI boot-up splash, followed by some version of Windows 10 initializing, connecting to some arcade VPN, and then booting the game. At my wife's friend's ice cream shop which has its own attached arcade where a day pass gets you all the games you want, there are virtual pinball tables which I've seen boot similarly.
It is, indeed, just a PC with a custom software stack built on top of Windows. Some old-style arcade machines you see in the wild these days are really just PCs, or maybe ARM SBCs, running a library of ROMs via MAME, legally or illegally!
There are no hard cut-offs, but contemporary systems are rarely implemented. MAME's focus on accuracy to a low-level degree means that most modern systems would be painfully slow to emulate.