Linearity of OTAs is defined the same way as linearity of any other components: By comparing the actual with an ideal transfer characteristic. In case of an OTA, it's done for the Iout versus Vin characteristic.
Limited linearity is typical for the standard OTA circuit due to the characteristic of the differential pair which determines the transfer characteristic. You should refer to analog IC textbooks for details.
Linearity is generally required for signal processing systems, not only for filters. If you are designing signal processing systems, you should know about the linearity requirements of the application. They can be given in different form, e.g. acceptable harmonic distortion (THD).