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.