There is a properly set keep alive by default for Particle SIMs, but to impose a 30 second ping in Safe Mode unconditionally would be harmful to Particle SIM data usage (default is 23 minutes).
To be honest, I don't think a user set ping in Safe Mode would be effective until the APN is recalled from flash memory for 3rd party SIMs. Without that, you won't even have a data connection. There is a case where you connect using your user code settings for a 3rd party SIM, then reboot into safe mode and it connects like magic still. This is because the modem doesn't get reset or powered off by default when the STM32 is reset via RESET button, or software reset (DFU mode, Safe Mode, etc..). But if you cold booted into Safe Mode with a 3rd party SIM, currently you could not make an initial data connection.