This is is the initial release of the "Flexmark (Markdown) Formatter" Jenkins plugin, which aims to replace the old "PegDown Formatter" Jenkins plugin that has not been updated for 9 years now. The PegDown Formatter is also known to be vulnerable to XSS attacks.
The used Markdown processor is Flexmark that is giving this plugin its name. Flexmark is basically a new implementation of the PegDown Markdown processor that has been deprecated in favor of Flexmark after. In Flexmark, XSS attacks are also prevented by default, see vsch/flexmark-java#221
Future releases of this plugin might implement more options for Markdown processing (feedback needed).
We are aiming to release this plugin as a plugin that can be installed directory from Jenkins very soon.