AFAIK, there is only ONE VFD that has achieved any type of approval for their safety circuit, the Schneider ATV71 (which is also the Toshiba AS1, but Tosh. doesn't have the same safety approval). Their "Power Removal" feature meets an EU standard, EN 954-1 category 3 and EN 61508 SIL2. Keep in mind, these are not power removal standards for personnel safety while working on electrical equipment and should NOT be construed to be the same as real power removal, even though that is the way the French translation came out for their sales brochures. It is the equivalent of CONTROL power removal, not line power removal. All SIL2 says is that there is a lower risk of inadvertant starting than "normal" (SIL2 is only the 2nd level out of 4, with 4 being the least likely). Schneider attained that rating by showing a low likelihood of catastrophic multiple component failures necessary for operation of the motor and a high degree of stability in the control circuit lockouts necessary to prevent the inverter firing the transistors in such a way as to make the motor turn. Keep in mind that a VFD has 2 stages of power delivery, conversion and inversion, so a lot of problems need to happen on both sides to make the motor turn. Like all other drives however, it still has the inherent risk of component failure from a line power isolation standpoint.
Soft starters of course all have that problem as well. In addition, the SCRs don't need to be "forced" to fire, they can (and often do) self commutate due to line noise, spikes etc. That, combined with a higher risk of component failure that could result in operation (remember, there is only one stage of power control in a soft starter vs a VFD), leaves soft starters much less able to attain any kind of SIL level approval. Nobody that I know of has even attempted to attain one yet, the concept is too far fetched.
A good point brought up earlier however is the issue of using a circuit breaker with a shunt trip as the SCPD of the soft starter. I know that Ford Motor Co., who has set a lot of machine power control safety standards over the years, will accept soft starters with a line isolation contactor and a shunt trip circuit breaker tied to a shorted SCR detection circuit as a safe combination package. I heard that Siemens is working on integrating a soft starter into a motor protection switch as a single unit so that they can open the disconnect portion in the even of a fault and this may allow them to get SIL1 or 2 if they pursue it, but I have not heard if they are doing so.