To give some brief update ... johnpolasek asked about ferrite cores. I have tried them with no success. However, there are different kinds of ferrite cores available and it is unclear to me how best to install them. Do you wrap the L1 and L2 wires individually around separate cores or together? How many windings? Somewhere I read the cores should be installed between the pump controller and the VFD motor, which is not possible with Pentair pool pumps.
From all the reading I did, a good EMI filter should work. But most filters do not attenuate enough in the 132kHz range of Insteon frequency. The good ones are very expensive. Most filters are designed for blocking powerline noise from entering sensitive electronic equipment. It is unclear to me how well they work in reverse, which would be the application with a noisy pool pump.
I had some success with the Intermatic ET-NF RC snubber.
https://www.intermatic.com/en/timer-controls/accessories/et-nfAfter installing it at all three of my pool pumps I no longer have Insteon communication problems if only one pool pump is running. Running more than one pump simultanously still gives me about 50% insteon command failures. So the Intermatic device seems to help a bit and at around $10 is not expensive to try out. For those people with a noisy irrigation pump or heat pump it might be worth to try.
Also, if you have only a few Insteon devices not responding, you can setup some triggers in Indigo if there is a device not responding to an On or Off command. The trigger action can send the command a few more times, which in most cases should do the job.