To put hypothetical dates on this, if I'm out driving tomorrow and I break the speed limit and I'm "observed" doing so by a camera and I'm not stopped at the time of the alleged offence, then the registered keeper of the car I'm driving needs to be informed within 14 days, tomorrow is 3 January, so by 17 January at the latest. Which means the notice can be served by First Class post if posted on 13 January or earlier.
If it's posted on (or before) 13 January but does not arrive until after 17 January then, if the registered keeper can prove this, the offence can not be prosecuted. However if the recipient can't prove this, the sender is allowed to assume that the notice was delivered in time and the offence can be prosecuted.
It can't be argued that this might be the case because of disruption and delays, but it can be argued that it is the case if there is supporting evidence to the claim of late delivery. Which is more likely at the moment than it used to be.
The legislation which supports this is unchanged because of the current disruption, clearly.
EDIT Note that the legislation also supports the hand delivery of the notice in person to the registered keeper within 14 days, but does not allow the use of Second Class delivery using Royal Mail because no presumption of delivery can be made using this service. Recipients sometimes wrongly assume that Second Class mail has been used when in fact First Class mail with discount for bulk mailers has been used, if you really want to get into the weeds here.
Bottom line - if you're "caught" speeding at the moment then it'd be useful to have a witness prepared to testify to support a claim of late delivery of First Class post if you're looking for a legal angle to "get off" the charge. However it would appear that most police forces post out the notices reasonably well in advance of the deadline already, to be on the safe side. "Our" (not "my") most recent (Greater Manchester Police) was well within 14 days.
FURTHER EDIT If you want to take advantage of this presumption of delivery, post items at the Post Office and request a free certificate of posting at the time you do so. This will record the date of posting and the recipient for you.