The venerable Masi Giramondo came w/ IS brake mounts. Nothing wrong w/ IS and there are still current drop bar bikes such as the excellent Black Mountain La Cabra that use IS. But adapters are required for IS and installation can, in certain contexts, but a bit cumbersome.
Flat mount was developed/promoted by Shimano circa 2014. Standard was focused on a cleaner, more "aesthetic" mounting standard for disc brake road bikes that would do away with stacking adapters. Shimano's considerable mainstream prowess has resulted in flat mount becoming the de facto standard for most current/forthcoming disc brake-equipped road, gravel, adventure, touring, and other sub-niche drop bar bikes. The cynic might argue that flat mount was just another example of the mainstream bike industry's love affair w/ "obsoletism." The optimist might counter that it's a better option in some contexts and there are still plenty of brake calipers made in the other disc standards should a frame maker want to go "semi-retro."
In terms of rotor size, my experience is that a good, properly set up disc brake system w/ 160mm front/rear rotors is more than sufficient for loaded, hilly all road touring (meaning pavement, gravel, dirt roads that aren't too rough). Still, should you own a flat mount bike and are pining for a larger front rotor, there are now a few adapters on the market that enable you to run a 180mm front rotor w/ flat mount (e.g., the Discord adapter). I haven't used them so I don't know how well they work. One apparent caveat is that you are advised to check w/ the fork manufacturer to verify that your flat mount designed fork can handle the forces associated w/ a larger (>160mm) rotor. Likely not a problem for a burly steel touring fork, but something to think about w/ the plethora of lighter carbon "adventure" forks now on the market and coming stock on many production bikes.
