By default if this is not specified in the engine JSON, it defaults to the MTS auto-calculated maxSafeRPM (above maxRPM when said variable is 2500 or lower, is calculated differently when maxRPM is 15k+)
However, if one wanted to make an electric motor, or a slightly more durable and/or realistic ICE, they could add and define a maxSafeRPM manually.

(example image)

Surprised it took this long for someone to make this ticket. I'll do my best to get this in.

Added in V19.15.0.