Using Neon 5.19 User Edition, Folder View configuration does not have "Show files associated with the current activity", is that something Kubuntu added?

You are right, that function disappeared from the list in Folder View in KDE Neon (I use Plasma 5.19.3) . Since when, I wonder? It is one of the most productive functions there is, I use it to have different sets of documents for six activities I have (which stand for six projects I do), using only one Folder View widget. Had not noticed this before, because my setup still works. I had a look at Kubuntu 20.04 (Plasma 5.18.5): There this function still is in the list.

There is a workaround, though. In Dolphin change the location above to ' activities:'. You get a list of your activities (the last one is 'Current activity'). Add the list to Places. Get a Folder View widget, go to configuration, and choose 'Custom location'. You get the Select Folder -dialogue with the Places to the left. Choose the item 'activities' in Places left and 'Current activity' in the rightpanel. The custom location is now 'activities:/current' and that does the trick.

Now that I discovered that: you can simply type activities:/current in the Custom location - box . That should work too.

Have a nive day.