If I install a self-signed certificate on my iPad with a validity period of 3 years, etc., does that mean that after 398 days, the certificate becomes invalid?

Re-read the article.

That lifetime limit does not apply here. Specifically, due to this detail: “This change will affect only TLS server certificates issued from the Root CAs preinstalled with iOS, iPadOS, macOS, watchOS, and tvOS.” A self-signed certificate or a local (private) certificate chain does not fall under that.

As for your question, any commercial cert with a lifetime past the specified limit is rejected.

If you want a ten-year or twenty-five-year self-signed cert, at least as far as Apple platforms are concerned, have at.

If you go that way, and for ease of private root renewal, I’d read up on the certificate serial number mechanism.

It means the certificate will be considered invalid and will be rejected, entirely, initially, eventually always.