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Electrolytic capacitors are still capacitors.  Of course no physical capacitor is ideal.

The charge as a function of voltage is reasonably linear for most electrolytic capacitors.  If it were not, I'd expect the datasheet to make that clear and provide some guidance about what to expect at different voltages.

Datasheets might express the non-linear nature of the charge as a function of voltage as the apparent capacitance as a function of voltage.  That's another way of expressing the same thing, although you have to be clear about whether that's the total effective capacitance, or the differential effective capacitance at each particular voltage.

Some ceramic dielectrics store less incremental charge at higher voltages.  That is often specified as a minimum guaranteed capacitance at the maximum voltage, or the drop in capacitance from 0 to full voltage.  Datasheets don't usually go into much detail because such caps are intended for applications, like bypassing the power feed of digital chips, where some variation in capacitance doesn't matter.

As always, read the datasheet for the details.