The latter rule is still the case and it was in effect the reason for the 10 year rule, which allowed the backdating of relief iover a period of time to a maximum of 10 years if going forward was not an option i.e. at retirement.
The 10 year rule was replaced and you can now only backdate AVCs to the previous tax year, but onyl if the payment is made by 31/10 in the current year.