This is something that has been puzzling me too. There remains a link to 'Link/view your O2 accounts' but it just takes you to https://accounts.o2.co.uk/ which doesn't allow any such thing these days.
That page seems to be all about bringing O2 products and services together under one merged o2.co.uk account and getting rid of the other o2.co.uk accounts completely. That is a good idea but it is a different concept to the linking/unlinking of accounts and it also overlooks the history of o2.co.uk accounts. It was never necessary to have O2 products or services to have an o2.co.uk account and they could be created just to have O2 e-mail address, and they could be linked to O2 broadband accounts as secondary e-mail accounts.
For my part I have two basically redundant accounts that were linked to my main account to give me a single point of access. Now I have no way of accessing them at all. It is not a great inconvenience as if there was a 'delete' button I've have destroyed them ages ago. However they may or may not contain personal information which may or may not be correct so I ought to be able to maintain them. If I were using them as secondary e-mail addresses I'd be unable to change the username and passwords on them because I can't unlink them.
G.
