Kali changed their state law, nothing new federally I am aware of.
What may be confusing things is ATF is not entirely forthcoming in their FAQs and are often outright wrong and misleading. Note the following:
"Receivers that meet the definition of a “firearm” must have markings, including a serial number. See 27 CFR § 478.92 (Firearm manufacturers marking requirements)."
That answer is from their "80% blank" FAQ. Sure sounds like as soon as Joe Gunowner cranks out an AR lower on his drill press he needs to stamp a number on it, right? Yet, if we go to the regulation cited in the answer we find this:
"How must licensed manufacturers and licensed importers identify firearms, armor piercing ammunition, and large capacity ammunition feeding devices?
(1)Firearms. You, as a licensed manufacturer or licensed importer of firearms, must legibly identify each firearm manufactured or imported as follows:"
Note the law specifically singles out licensed manufacturers and importers, it is silent about everybody else. The requirement to put on a serial number only applies to people with FFLs, not the guy building his own.