Project: QGIS
Version: 3.32
Instead of limiting layout legend filtering to a single linked map, this change permits legends to be filtered instead by multiple linked maps. It is designed to accommodate the use case where a layout has multiple maps, potentially at different scales and showing different extents, and a single legend is required which includes all symbols visible across all the maps.
The UX has been designed to avoid changing the current user workflows, in that legends will still be linked to a single main map item. This map is used when determining the scale (and other map dependent properties) at which to render the legend's symbols. Checking the "Only show items inside linked maps" option will still automatically filter the legend by the content of the main linked maps. However, there's a new "..." options button next to the "Only show items inside linked maps" checkbox which allows users to select also other maps from their layout to consider when determining visible items:
