Just got this thought.
Once a specific rule added onto ASA interface, the default Any to Any less secure networks ACL rule is gone.
So is it even possible to add the "Any to Any less secure networks ACL" back on ASA? via ASDM and/or CLI?
This is the behavior if you have no ACL applied. ASDM probably lists this just to be more clear about what the default behavior is. If you delete your ACL or remove it from being applied it should fall back to this. If you're still having issues with it post the output of your show run from the CLI.
So if I deleted my added rule, the "Any to Any less secure networks ACL" should be back automatically?
Yes. That should be the case. The rule is implicit and doesn't really exist. It is just shown in the ASDM to clarify the behavior.
I will try it tomorrow.