I think LevelUp has a good replacement for legendary resistance: Elite Recovery. I have come up with a few versions of the concept myself, but ultimately I have kind of standardized on Elite Recovery. I also simple give them the appropriate saving throw proficiencies as well. I do think WotC is changing the wrong Legendary marker.
Elite Recovery as a Legendary Action:
Elite Recovery. The dragon ends one negative effect currently affecting it. It can do so as long as it has at least 1 hit point, even while
unconscious or
incapacitated.
Elite Recovery as a Trait:
Elite Recovery. At the end of each of its turns while bloodied, the pit fiend ends one negative effect currently affecting it. It can do so as long as it has at least 1 hit point, even while
unconscious or
incapacitated .
Of course LevelUp also gives monsters Legendary Resistance, so maybe they don't see it the same way!