"Phone numbers, email addresses, birthdays, and marital status "
At the same time the IDPC might ask FB and the rest why birthdays and marital status is held. I understand that there may be requirements to know that an individual is over a certain age but that's simply a Boolean parameter, a date is not required.
Perhaps el Reg could also ask them. Lately Google has taken to nagging about needing a date of birth on "my" phone account alleging this to be a legal requirement but somehow overlooking any citation of the statute or explaining why it things this can only be satisfied by a date. Given that no actual name is attached to the account it's a little OTT but it seems, at least for the time being, satisfied with the date of the start of the Unix epoch .