I did a video on this previously, I think it was over a year ago now. Back then there was a Live chat and and option to delete your account directly from the portal. It was this option that helped me unlink my account after weeks fo trying. By deleting my duplicate account I was able to unlink an old xbox account so I could finally link it to my main account and be able to cross play!
However this is no longer available anymore and after exhausting all other support options the only way around this is to use the GDPR request portal and use the right to erasure option to delete and unlink your account.
This only works if the account you have is no longer needed. In the video I demo how easy it is to submit a right to erasure request. Once you action the request it can take 20 dasy or so for it delete, but I've seen it done faster.