New expression variables have been added for use in data defined expressions in layout legend items, including:
If the legend is linked to a map, then expressions used in that legend will also have access to the linked variables, including @map_scale, @map_extent, etc.
Additionally, symbols rendered as part of a legend now use the full available expression context for the legend. This means the symbols will have access to variables like the current atlas feature.
This feature was developed by Alexis Roy-Lizotte