Installing the GeoServer YSLD extension - GeoServer 2.19.x User Manual
The YSLD extension is listed on the GeoServer download page. To install: Download the appropriate archive from the GeoServer download page. Please verify that the version number in the filename corresponds to the version of GeoServer you are running. The file will be called geoserver-A.B.C-ysld-plugin.zip where is the GeoServer version.
https://docs.geoserver.org/latest/en/user/styling/ysld/installing.html
Seonglae Cho