I know the history, I was there and I've written (read: complained) about literally every iteration of Monk at length at this point so I know how it worked at literally every step and when I said "end of Shadowbringers" I meant "end of Shadowbringers", obviously not any of the other points in the patch cycle.
Here's a demonstration of the mitigation you got from Riddle of Earth that I did immediately after the patch dropped in 5.5. As you can see it scaled in that period per stack.

End of Shadowbringers Monk just had an off brand Sentinel/Shadow Wall, it was goofy as hell.
Current Riddle of Earth doesn't scale like that, it's just a flat 20% at any number of stacks. That isn't having three stacks of Sentinel crazy like it used to be, but it's still the best defensive action on a DPS in terms of both defensive power and the sheer amount of uptime you have with it in the game at the moment.