Flat res reduction and ways to apply (and how important is it still?)

For masteries it was done this patch. The default value for RR in masteries was 30% instead of 25%.

Yes, but it only works as an argument for increasing the abundance of flat RR, while my point was flat RR constellations are mandatory unless the build has it else where (which mostly pierce Blademaster, BWC build and to lesser extent builds that make good use of War Cry RR, mostly Warborn and Krieg), and not picking one is like not picking two T3 constellations worth of %damage.
One can increase the build variety by either increasing the abundance of flat RR or by nerfing it’s values so that not picking it was more of an option and nerfing enemy resistance to compensate.