Hello,
In experience builder, is it possible to show all items in a legend even if they don't appear in the map? I see options to show visible layers and show visible layers within current map extent but I would like all of my layers to appear in the legend regardless of whether they are visible in the map or not. I have arcade expressions for each layer so that the data only appears on the map based on a timestamp but I would like the user to know how the data will appear if those arcade conditions are met.
Currently there is not, your options are visible layers or visible layers within map extent like you noted. You would probably need to modify the out of the box widget with Developer Edition.
My current work around is to use a Filter widget with filters on each of my data subsets (arcade expressions in my webmap that only display specific time frames) and download icons from here with my line symbol colours. The only downside I see so far is that the toggle button is backwards. When it's flipped to the "on" position the data disappears from the map.
Sounds like you are well on the way to a great solution. The next steps would be to write your filters backwards (turn IS into IS NOT), link them together with an OR and then use the Apply This Filter Automatically Option. If my logic is correct, this should make turning off a filter turn off a style.