The Arm processor requires that
reset is kept low until the power is stable. I suspect that the Pis honor that requirement.
For power-down, the ARM says:
6.3.2 Power-Off Sequence
The power supplies can be powered-off in any order as long as LVCMOS supplies operated at 3.3V
(DVDD3318_A, DVDD3318_B, or DVDD3318_C) never exceed static 1.8V supplies by more than 2 volts.
There is no specific required voltage ramp down rate for any of the supplies (except as required to meet
the above mentioned voltage condition).
So the main processor should not be a problem. You may want to investigate for the specific memory card that you use.
Note that the diagrams for the Pi are available at https://www.raspberrypi.org/documentation/hardware/raspberrypi/schematics/README.md. You can see the additional power circuitry there (f.e. the processor runs of 3.3V, not 5V)