This feature allows for expressions to be embedded directly inside legend item text (e.g. group, subgroup and item text). The expressions are evaluated at render time, with full knowledge of the legend's expression context (so can utilise variables from the layout/layout item/etc)
There's no UI for this yet (that will come in 3.8), but expressions are entered using the standard [% 1 + 2 %] format.
E.g. a legend item text of
My layer (rendered at 1:[% @map_scale %])
will show in the output as
My layer (rendered at 1:1000)
This feature was developed by Nyall Dawson (North Road)