I received an email stating Update your sign-in technology before September 16th, 2024 to maintain email access. Is this legit or a scam?

It wasn't offensive at all. Just careless. You're an Outlook MVP and a moderator. People trust your advice. If you tell them it is real. They will trust your advice and if it happens to be a phishing mail that looks like the original notification, they're doomed. I'm sure you know there are many phishing email that looks very similar to an original Microsoft mail.

The email is about Outlook.com and modifications that may be necessary to email programs that connect to Outlook.com.

Modern Authentication Methods now needed to continue syncing Outlook Email in non-Microsoft email apps

I'm sorry I offended you. I have deleted the offending post; you may rest easy. Please feel free to reply to the OP in an appropriate fashion.

That's irresponsible reply. Yeah there's a real notification from Microsoft about that but what makes you sure Thomas did not get a lookalike email with a link to phish his password? He sees your name, an MVP, trusts that link and follows it (click this link to update your security). Got his account hacked. Now what?

You can't rule out that scenario by answering "real, not scam" when you don't even know what he got.

I am confused. I have multiple emails and use the latest Outlook 365 desktop version on my laptop and have all of those email addressees connected to it. Some of my emails are from my ISP and are connected to my Outlook with POP/SMTP. In fact, one of the POP emails is where my Office365 subscription resides and Outlook profile is attached to. What do I do with those? Do I lose those?