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."
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":
tdf#124606 Layers should be under sdraw/