The Worldborder is no longer visualized by particles. Actually, the border should be shown to the player with flame particles just before he meets it ((https://imgur.com/a/yg022Jy). Since I went from version 1.13.2 to version 1.15.2 this feature does not work anymore. No config settings and not even the Worldborder version has been changed. The Worldborder is just completely invisible.

Paper 1.15.2 – 220
WorldBorder 1.9.10

It was not a Worldborder bug, but a bug in the visualization using the SelectionVisualizer plugin. Worldborder has never had such a feature. Sorry for the inconvenience!