in blender 2.8 the rig layer menu appears when pressing the "N" button, but where can I find the rig layer in blender 2.9
Open a "text editor" window, select a file that should already exist from the dropdown menu (rig_ui.py) and press the triangle play button, tu run the script.
This procedure recreates the Rigify layer ui menu in the N panel, if for some reason it hasn't been created.
This can happen if the "Auto run Python scripts" preference is disabled, and/or if more than one rigify rig is created in the same blender file, and/or if a rig is linked/appended in a new file.
It's still there.
Make sure you have "rigify" Add-on ON, add the meta-rig (Add > Armature) and then you need to generate the rig. For more info about rigify here