There is no overlay on ext4-based virtual routers.

    # mount|grep ext4
    /dev/mapper/pve-vm--501--disk--0 on / type ext4 (rw,relatime,stripe=512)

The /overlay directory exists just like the /rom one does but they're empty:

    # ls -l /|grep -E 'overlay|rom' 
    drwxr-xr-x    2 root     root          1024 Jan 27 23:53 overlay
    drwxr-xr-x    2 root     root          1024 Jan 27 23:53 rom

These are empty directories (there is a note file in /rom with some info for those using squashfs which is not applicable to these installations).