What is the equivalent capacitive load seen by the IC if in a datasheet of a chip CLmax=10nF given and I connect an RC filter as a load?
If you connect an RC filter the equivalent circuit is R in series with C.
CLmax is specified because the load of a digital output is supposed to be a digital input which is a capacitive load (the gate to source capacitor).
If the load of that digital output is an RC filter than C may exceed the value of CLmax.