It used to be pretty common for RAM or peripherals to have wait states, or REQ/ACK handshakes, or similar mechanisms that would allow the peripheral to request

It used to be pretty common for RAM or peripherals to have wait states, or REQ/ACK handshakes, or similar mechanisms that would allow the peripheral to request a variable amount of time to respond. I suspect it's not a more popular scheme because it makes it hard to pipeline your activity if there's unpredictable timing somewhere.