The paper version of the application forms and guidance for applications made in the UK (FLR, ILR etc.) always stated that every single page of every passport submitted must be scanned, including all blank pages - a declaration also had to be signed stating that every single page of the passport had been included.
From the April 2018 FLR(M) and July 2018 SET(M) paper application forms (the last ones published before they were moved online) (the only application options at that time were by post or an in-person priority appointment):
If you are making your application by post you can send a complete and full copy of your sponsor’s valid passport, national identity card or travel document and any associated Biometric Residence Permit. Every page of the passport must be copied including any blank pages
And from the sponsor's declaration section:
I understand that I can supply a photocopy of my entire valid passport, national identity card or travel document if this application is being sent by post only. Where I have provided a copy of my passport or travel document, I confirm that it is a complete and true copy of the entire original document, including blank pages, and I will be physically present in the UK at the time this application is submitted. I understand that checks to confirm my status will be made. I understand that I may be asked to submit my original passport or travel document and or other evidence of my status at any time.
However, since the application process went online, there has been no new published guidance that can be viewed without going through the entire online application... and as we have not been made aware of any official changes to the passport requirements, unless it is now explicitly stated in the online application that only the non-blank pages need to be scanned, we will continue to advise that all pages should be included - if only to be on the safe side, and to avoid being contacted by UKVI to ask for more information... which can delay the processing time of the application.