None of the browsers require by policy that certificates be logged. What this means is that the existence of a certificate which wasn't logged is not by itself a misissuance. Whereas for example the Apple 398 day rule is a policy rule, so a certificate which breaks the rule not only won't work in Safari, but it is also a misissuance and your whole CA might get distrusted by Apple.

However, all the major browsers except Firefox require that certificates they are shown which purport to have been issued after a mandate are presented with SCTs. We'll discuss what that means below. For Chrome that mandate begins after 30 April 2018, which means it doesn't catch certificates issued in a small window of time when certificate lifetimes up to 39 months were still allowed at the start of 2018, the last of these certificates would expire at the end of next month, May 2021.

In practice no public CA was selling unlogged certificates intended for web servers by the point the mandate triggers, it would have been a needless business risk to sail so close to the wind, so chances are no certificates in this category exist today.

Signed Certificate Timestamps are issued by the log, they are like "proof of posting" when you send a letter. The log warrants that any certificates for which it has issued SCTs will appear within the Maximum Merge Delay (for public CT logs this is 24 hours).

That might seem like a long time, but it's a do-or-die promise. Logs which experience a problem making them unable to show a consistent log with the corresponding certificate within 24 hours are disqualified and you need to start over, because without such a rule obviously you can smuggle anything into an outage.

Google and Safari's policy (I don't know the Edge policy) dictates two or more SCTs, at least one to be from a log controlled by Google. So this gives Google the handy property that they don't need to trust any combination of third parties, you must show all certificates to Google itself.