Hello!
I used to apply this little trick of using a pie chart with only the legends shown to act as a page-wide legend to all my visuals shown. Here is an example:

To explain, i have all those graphs with different measures, which are results for different models. So instead of enabling the legend inside one of them due to size limitations, i'd add a pie chart with it that goes along the page width, to further improve the data reading for the report consumers.
Not only for visual appeal, this solution also provides quick filtering for the selected data since we could select the models we wanted to display - we have slicers to select what is shown, but they take a little more time, while selecting stuff straight from the "legend" is faster and keeps the audience looking at the data.
Well, today i saw my legends were gone. Last time i used them was friday, and they were fine (i use them on daily meetings) and my first thought was a field could be broken or an update on field interactions had disabled it. There's just an empty visual in there. It will show up again if i add anything to the value field, which i didn't use cause it would then show the graph again when i didn't need it, or shrink the legend to accomodate the visual. Doing this seems to work, but i'm still testing if it ain't gonna bring the pie chart itself again.
When i open the report in the Desktop app, it shows as it was - and i made sure to update it to the latest version as well, and the legends were displayed as well. But at the online app it doesn't show - i've tried reuploading it with both unupgraded and upgraded desktop app, none of them worked.
My guess is that since this trick is done with an empty pie chart - using only the legend - web app included a new optimization policy to keep visuals as blank if no value is shown.
That said, i'm reaching out here to either check what happened to this solution, if there's any other solution like a custom visual for page wise legends - and i'll add that using a graph "behind" all the one's ain't a good solution cause it will be pulled to the front if users interact with it, and i cant set a fixed legend cause the data is dynamic and i want it to be interactive with the visuals - or if that is a side-effect from a new feature and if there's the possibility of enabling it back on the online app again.
I also provided example of usage in the suggestion forum, asking for a page legend that interacts with every element (as long as the user provides the same field for every visual, obviously).
That's it, thank y'all in advance!