So, with 5.2 (or possibly earlier), it came.
So, for Heavensward, it starts at itemlevel 100 for Dusk Vigil, going up by 5 each time until i120 for Gubal Library,
For Stormblood, it starts at 240 for Sirensong sea, going up by 5 each time until 260 for Abania
For Shadowbringers, it's 370 for Switch, going up by 5 each time until 390 for Gulg.

Sorry if this counts as a necro, I figured since it related to this, it'd be worth mentioning in this thread, we finally got our request!