EA account can only be linked to one xbox account in its lifetime.

According to this EA Help Article:

When you link accounts, it creates a lifetime link to protect you from malicious activity and help prevent performance issues in your games.

So it's been implemented for security reasons, but also for some technical reasons.
Unlinking/Relinking platform accounts doesn't have that much impact within Apex anymore since cross progression came out, but keep in mind that platform accounts are linked to EA Accounts as a whole, so there likely are other games that would be affected by allowing different Xbox accounts to be linked.

Though, what is the exact problem that you are having?
Have you lost access to your Xbox account and are you trying to play the game under a different Xbox account?
If that's the case, I don't think much can really be done except trying to get access to the Xbox account that was linked to your EA Account.
I have seen some reports from users where EA Support were able to help them out, but those seem to be quite rare and would also seem a tad unlikely as it would go against what was said on the EA Help site.

Your progress should still be on your EA Account as a whole if your platform accounts were linked when Cross Progression came out.
So you could still access it via other platforms, except your Xbox account if you cannot get access to it again.