If you have no spf or dkim your emails have been getting rejected for a very long time.
(dmarc only requires one of spf or dkim, not both, but it is a good idea to use both)
Not IME. I just added all 3, after not having any noticeable email issues for years.
Unfortunately, SPF and DKIM become optional once you send enough email for a mail server to trust you. You could've never bothered to update your email to the futuristic year of 2015 and still made your way into every inbox without ever knowing you forgot something.