As of today (June 21), Roku pushed some sort of update that is only triggered by doing a manual update check. When I did this, it said "aligning solar panels" or something. It didn't actually do a system update, but when I then went to Screensavers, City Stroll Movie Magic was back and able to be selected. It's dated 6/21/2021, and the Movie Magic that comes up when you do a Search is still broken (no image and it says "Could not add" when you try to Add Channel), so they clearly broke this and just fixed it today.
In other words, there wasn't a system update needed, but doing a manual system update check will trigger whatever was needed to fix the City Stroll: Movie Magic screensaver.
Actually it did do an update - an update of the (hidden) City Stroll: Movie Magic screensaver app (yes, screensavers are apps).
Ironically, they "broke" this today: If you do a search and add it, it will add the older 30.0.35 version, instead of the newer/updated 30.0.40 version, and both will be listed under Settings/Theme/Screensavers (I recommend removing the old 30.0.35 version and keeping the newer/update 30.0.40 version).
NOTE: The above only works on Ultra models - all other models show a blank image/no info and attempting to add the channel results in "channel install failed"
This really has been a poorly explained & executed promotion of the Disney movie "Luca" where they decided to temporarily replace a very popular screensaver with the Luca one (obviously for the purpose of maximized exposure), and broke the search/install for it in the "reversion" process.