QGIS Server 3.28 is currently certified for WMS 1.3.0 (see OGC Reference Implementation).
In order to keep an eye on regressions, OGC tests are run on a daily basis on the master branch. Corresponding reports are generated for several OGC testsuites:
WMS 1.3.0 : http://test.qgis.org/ogc_cite/wms_130/
OGC API Features (aka WFS 3) : http://test.qgis.org/ogc_cite/ogcapif/
WFS 1.1.0 : http://test.qgis.org/ogc_cite/wfs_110/