If there's one thing these circuits are absolutely not it's being linear.
Every interesting circuit using op-amps basically is interesting because of the various non-linear behaviors you have to deal with when you optimize the circuit.
I find a x10 amplifier with high Zin, low Zout, low noise, low error, and constant delay to be interesting enough.
Every circuit is non-linear when used outside its design limits, but this hardly needs asserting.
And of course real devices are not perfect. Resistors have inductance and capacitance and a thermal coefficient, capacitors likewise have flaws, wires have resistance and self-inductance, and circuit layout gives you stray capacitance, crosstalk, and instability. Op amps themselves produce noise, have input currents and offset voltages and limited slew rate and bandwidth. And so on, and on.
But again, none of that needs asserting.
While you certainly can design lot of deliberately non-linear circuits with op amps and non-linear devices in their feedback paths, it's difficult to design linear circuits without them, or instead with circuits, the core of which end up looking like simplified op-amps.
So, what did you have in mind when you said "the various non-linear behaviours"?
None of the characteristic curves of vacuum tubes and transistors would make you think that there was anything linear about them. However, passive components such as resistors and capacitors are quite linear and stable. So the trick with feedback is that the feedback network defines the response of the circuit despite using potentially nonlinear and unstable gain elements. This was of vital importance for making important circuits, like amplifiers for telephony, predictable, reliable, and reproducible.
If you design something like a basic analog filter using op amps, and model its behavior under the assumption of linearity, the actual behavior of the circuit will resemble the model behavior to an uncanny degree.
But naturally, terminology is not precise. For instance the term "linear" is used to describe things like power supplies and voltage regulators, in contrast to "switchmode" circuits. A "linear power supply" means a particular thing that has little to do with linearity.
Fully aware.
What I'm saying is: op-amp based-circuits just never do that in the real world, only in extremely limited circumstances (hence the whole small-signal model methodology).
They are in general anything but linear.
RF design is the darkest of the arts.
It's not nearly as mystical as folks like to make it sound. A lot of the language around it dresses it up to appear more than it is in an effort to be rigorous.
Systems that are both linear and time-invariant, sometimes abbreviated to LTI systems, are the most common sort of thing (i.e., model) that one actually analyzes. When time-dependence is present, it is always modelled explicitly and usually modelled as a complication of a time-invariant system.
Not sure where you were taught that, but it does not strike me as the generally accepted meaning of the term "linear".
Where I hail from, linear means:
f(a*x + b*y) = a*f(x) + b*f(y)