A DESCRIPTION OF THE PROBLEM :
java.util.Date is used as the return type of these functions which makes it difficult to use if the rest of the system is using java.time. You'd need to convert Date to Instant somewhere in your system which can cause errors and bugs. For something security related like X509Certificate I don't want to have to convert between Date and Instant.
In general, it is a bad idea to keep using java.util.Date in the Java API, but especially so in security related API's.