>Security [bolts] are used to stop Tyre creep for what other reason are they on there used especially on trials machines as the tyres are run at low pressure
"low pressure" being the operative phrase? There is less pressure squeezing the tyre beads against the rim. Otoh, rear road tyres have been run either close to or over 30 psi for decades.
>What amazes me now is that modern tyres don't need them even with the high output of power on modern bikes?
... because, as I say, modern bikes run higher pressures, plus tubeless tyres?, plus the modern rim profile grips the tyre bead differently.