Reader's Digest version of what the issue is.
In years past, the magnet wire used to make motors was made to handle at least 2x the expected line voltage. With the advent of VFDs, the way they work can create a phenomenon called "reflected waves" or "standing waves" in which the VFD output interacts with the wiring causes brief but repeated "spikes" of voltage that can, in theory, reach 3x the line voltage. The longer the wires are between the VFD and the motor, the worse this becomes but conversely the shorter the wires, the less it is a problem.
But in reality, a motor mfr that makes 230V and 460V rated motors does not use two different ratings of magnet wire, they used one, and it was rated for 1000V because that was more than 2x 460V. A couple of decades ago, many motor mfrs changed to using 1200V insulation in order to have only one supply of magnet wire for when they made 575V motors for Canada. More recently since VFDs are becoming ubiquitous, some motor mfrs just go ahead and use 1600V or even 2000V insulation and keep everything the same. But still, even 1000V is more than 3x the 230V level, so the potential damaging spikes of voltage from a VFD are still well within what the insulation can handle.
There is more to it than that of course and if your motor insulation is already compromised by age, contamination, moisture intrusion etc., using a VFD may accelerate the demise regardless. But what I tell people with 230V motors is that you ALREADY own that motor, so there is no reason to run out and change it prophylactically; just use it as long as you can, THEN if it dies on you, replace it with something else.
If the motor is special and / or hard to change out, then you can consider adding what's called a "dv/dt filter" or even a "sine wave filter" to the output of the VFD. But in many cases on small motors, the filter may equal or exceed the cost of the motor, so you have to weight the risks / rewards.
