Accessing old SA302 for HMRC Public sector Pension Adjustment Service

Hi,
I am trying to get a calculation through use the HMRC Public sector pension adjustments service.
This requires me to input my total income for tax years from 2015-16 onwards.
The calculator and the accompanying how to video says that you can get that information fro the SA302 if you complete self-assessments.
However I can only access SA302 from 2019-20 onwards.
In other threads is looks like you have can only access older SA by making a request in writing.
Is that the case and if so why is this not explained on the calculator and the how to video guide?
Many thanks

Hi Peter Overton
Please see the guidance at: Get your SA302 tax calculation.
While you can view and print SA302s from up to 4 years ago via your Personal Tax Account, older calculations need to be requested in writing.
Please see: Self Assessment: general enquiries
Written requests can be made to:
Self Assessment, 
HM Revenue and Customs, 
BX9 1AS, 
United Kingdom.
If you feel that the information should be included on CCalculate your public service pension adjustment then please provide feedback directly to GOV.UK. 
You can do this by clicking on Contact at the bottom of the page and complete the GOV.UK contact form.
The Customer Forum cannot do this on your behalf
Thank you.