I can report that the slider works fine in Debian Buster (Tor browser 9.0.2).
How did I determine this?
Went to a javascript heavy page with the shield black. Page loads.
Again, reload the same page, but this time with the shield completely clear and the page took a bit longer, plus many more graphical decorations were present.
Tried several sites in total, including this one, no problems reported with the Tor Browser.