Most synths have one or more LFOs or Low Frequency Oscillators. These can be used to modulate many different things depending on the synths capabilities, but there is generally an option to map the lfo to cutoff or filter frequency.

Also, if you are working on general audio and not just within a synth module, you can record automation and adjust the cutoff knob of your filter as necessarily. Or you can draw in automation in a repeated pattern (a simple triangle wave would be pretty easy to draw)

And lastly, if you want to escape VST filters altogether, you just need to understand that the cutoff knob you are playing with is just a LPF or low pass filter. I don't know if you are familiar with EQs or not, but this is where you designate a frequency and bandwith (or 'Q') and all frequencies above the designated one will be attenuated or cut entirely. The shape of the curve depends on the gain and Q setting. Therefore, if you want to create this effect without using a VST filter, simply pull open your EQ, turn on the band on the far right (high) side of the graph, change it from notch or high shelf to lowpass or highcut. Then you can automate the frequency of that EQ band and achieve the same effect.