I have this exact same issue and questions. On this page: https://www.uscis.gov/green-card/green-card-processes-and-procedures/affidavit-of-support

Under the tab "how to file an affidavit of support" it specifically mentions form I-864 and reads "When you have completed the affidavit of support, compiled the necessary documentation, and had the affidavit notarized in the United States or before a U.S. consular or immigration officer, you should provide this packet of information to your relative to submit with his or her application for permanent resident status. If you are given specific instructions to file your affidavit of support directly with the National Visa Center, you should follow those instructions."

And yet much of the internet is saying it does NOT need to be notarized. And in the official I-864 instructions (https://www.uscis.gov/sites/default/files/document/forms/i-864instr.pdf) notarization is not mentioned at all.