From v9.5.2, swatches are deprecated and discontinued in favour of showing the first (if any) of a notes defined flags ($Flags), as described here.
This feature and its associated system attribute are now essentially non-functional.
Outlines can show a square colour swatch, including border and pattern information, in the area reserved for the link widget between the note's icon and its badge. Because of the swatch's positioning, when a note with a swatch is selected, the link widget temporarily replaces the swatch.
Swatches are activated via the $OutlineColorSwatch attribute.
This feature can be toggled on/off via the Appearance Inspector's Outline tab.