It's a standard that is "enforced" in two places I think:
* Place 1: ISPs configure their carrier-grade routers to drop it if they receive it.
Technically, there really is nothing stopping an ISP from actually routing it, just like there is nothing preventing you from assigning an already-assigned address like one of Google's to a NIC in your LAN.
But ...
* Place 2: Internet registries won't give out IP blocks in these ranges. So no ISP can make an AS that has routes for these addresses.
Technically again, there really is nothing stopping a registry from assigning something in the block. Imagine if someone bribed a registry to give them out. These assignments are basically what makes the global BGP table that is the "top layer" routing information for the Internet.
But:
A) all registries would have to cooperate at once for it to be globally reachable - ARIN, RIPE, APNIC, AFRNIC, and LACNIC - and current geopolitics would almost guarantee that wouldn't happen. IPv4 space is scarce and valuable, so they'd really fight over it, and
B) everyone is already using it for private IP space anyway, so even if you hosted something on a publicly accessible 192.168.1.1, no one would be able to reach it due to existing configs and assumptions, so it would be pointless.