It seems that ceiling of Rs. 20 lakhs is applicable for those employees who retire on or after 29.03.2018.
It is my understanding.
Others may clarify.
In the last occasion
( Ceiling raised from Rs.3.5 lacs to 10 lacs) also the date of effect was 26.04.2010
(date of passing the bill in parliament/ Gazette date) and not from 01.01.2006.
i.e pay commission date.