That doesn't require you to deliberately assign a new IP every time the router reboots, it just requires you to be able to re-use IPs without an active DHCP lease.
In practice customers don't usually turn their routers off for very long, and many ISPs don't have an acute shortage of IPs (those that do have already moved to CGNAT), so it's pretty typical to keep your IP no matter how many times you reboot your router. If I'd leave it off for a month I'd be less sure I'd get the same IP.
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That doesn't require you to deliberately assign a new IP every time the router reboots, it just requires you to be able to re-use IPs without an active DHCP lease.
In practice customers don't usually turn their routers off for very long, and many ISPs don't have an acute shortage of IPs (those that do have already moved to CGNAT), so it's pretty typical to keep your IP no matter how many times you reboot your router. If I'd leave it off for a month I'd be less sure I'd get the same IP.