This seems overly dismissive especially for individuals who send emails from custom domains at low volumes. DMARC has previously been seen to only be really needed for high-volume senders. The mail testing service linked still gives 10/10 for emails received without DMARC (but with SPF and DKIK). My email is setup via Fastmail and I followed all the recommendations they provided (which didn’t include DMARC)

Imagine that I deploy a site in HTTP-only, without TLS, and then complain "hey, why are browsers claiming that my site is insecure and showing some red padlock shenanigan, scaring my users away? Am I supposed to go around the Internet, checking what each browser considers 'secure' or not?"

I've had DMARC implemented on my low volume server since I learned about it 10 years ago, it's not that hard.

I just tried this out now with an email from my personal domain that I have configured to send via my gmail account. I got a 10/10, but DMARC does not seem to be required for this. It shows a yellow checkbox for me on this, the same as it shows for me not having an unsubscribe button, which hardly gives the impression of it being necessary, given that I would not expect an unsubscribe button to make sense for most people manually sending emails to their customers from corporate email accounts.

Not enough. O365 still discard/lose e-mails.

This service is not accurate, it cannot handle multiple dkim keys.

The funny thing is, my domain with Fastmail gets 10/10 and I didn't have a DMARC policy configured. So you're being dismissive when you don't even know what you're talking about.

How about you read other testimonials in this thread and dial down your patronizing mouth?