Can I assume input protection diodes in all modern ICs, even when not mentioned in the datasheet?
Modern ICs cannot survive without ESD protection so you can assume that all ICs have this protection.
However there are of course exceptions:

Not all ICs have input protection on all pins, for example a current sense input might be made such that it can sense voltages above or below the supply voltages.

Not all protections consist of diodes to the supply rails as in the classic ESD protection scheme:



Source: https://semiengineering.com/how-robust-is-your-esd-protection-are-you-sure/

Some ICs use ESD protection not like the classic diodes - resistor - diodes like shown above but instead use a &quot;grounded gate NMOS&quot;. Such a device behaves like a fast zener diode. Since this grounded gate NMOS is connected between an input and ground (the ICs substrate) there is no diode to the positive supply rail.

Some ESD protections rely on sudden increase in voltage and these will not clamp / protect against a voltage coming in from an input pin.


So the safest bet would be to assume that the IC has no protection at all and that you need to clamp the voltage and limit the current. I would use a series resistor and then some diodes, zeners or TVS so that you can be sure that in input voltage rating of the uC is never exceeded.
Also, I would consider it bad design practice to rely on the ESD protection of an IC to make the circuit work and/or protect itself. If needed I prefer to have an external circuit to protect against unwanted input voltages and currents as you can never be sure how the ESD protection on a chip will behave. This is especially true if a manufacturer writes the datasheet as if the protection does not exist (no max. input current specified) as then you have no guarantee that any current that might flow into an input, is OK.
And remember that for proper input protection you always need to do two things:

limit the voltage (the diodes in the schematic above)

limit the current (the resistor in the schematic above)