All our stuff about layers are under simpress: https://help.libreoffice.org/latest/en-US/text/simpress/guide/layers.html?&DbPAR=IMPRESS&System=UNIX The About page says this, apparently as an attempt to clear the confusion: "Layers are available in LibreOffice Draw, not in LibreOffice Impress."
Confirmed. Moving help pages from /simpress/ to /sdraw/ will trigger retranslation. TM at help here.
Olivier Hallot committed a patch related to this issue. It has been pushed to "master": https://git.libreoffice.org/help/+/a96a32ca8505d7f2bd5a763572c5624ae755e991%5E%21 tdf#124606 Layers should be under sdraw/