It all references 2015 because it was written then, it's tied with the rolling 40 year exception.

https://insidedvla.blog.gov.uk/2015/11/17/whats-the-story-with-black-and-silver-number-plates/

Vehicles that can display black and silver plates

Since April 2015 vehicles manufactured before 1 January 1975 can display the older style plates. You must:

have applied to DVLA, and be registered within the ‘historic vehicles’ tax class. This allows your vehicle to retain its authenticity and be in keeping with its age.

Vehicles constructed 40 or more years ago are exempt from vehicle tax.

The 40 year exemption date rolls forward automatically each year on 1 April.

The " Since April 2015 vehicles manufactured before 1 January 1975 can display the older style plates." is linked to https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/293910/TIIN_2507_8011_40_year_rolling_exemption_for_classic_vehicles_.pdf

In which it says:

The measure will have effect for eligible vehicles presented for exemption from 1 April 2014 and each subsequent 1 April thereafter.