Irregular last bar

Where in the documentation do I find how to make a last/final irregular bar, when there is a pickup bar at the start? TIA.

There are several ways – possibly the easiest in Dorico 4 is described in the tip at the end of this page (which I think I shared with you earlier today?)

As described on that page I get a new time signature (which can be hidden) for the irregular bar when I delete the rest.

If I place the cursor on the first of the rest(s) to be removed and insert a final bar line the result seems to be the same, minus the time signature showing up.

Are they both equal methods?

Reckon so. If you want Dorico to hide automatically any time signatures changed as a result of using the Insert Global Adjustment scope, change the corresponding Note Input Option, as mentioned in the next paragraph.