What does the "online only" processing of refund instructions that was put in place on 1 October 2019 mean?

It means that refund instructions can now only be filed in an electronically processable XML format using one of the electronic means of filing listed in point 15.2 ADA. Refund instructions submitted on paper, via the Webform Filing service, as a PDF attachment or using the annotation field in the online forms are invalid and will not be processed. As a courtesy service, the EPO will inform you about invalid instructions and ask you to claim any refunds online at fee-payment.epo.org/en/login.