Account linking will soon be discontinued
To help increase account security, the option to link multiple accounts will be discontinued.
Your accounts will remain unchanged, but to switch between them, you'll need to sign out of one and sign into the other. Also, the account menu in the upper right of Outlook.com will no longer list your multiple accounts.
To send and receive all your mail from all your accounts in one place, see Send and receive email from other accounts in Outlook.com
It seems as time goes on, various services persist in cutting back on functionality. Linked accounts was a useful feature for people who have multiple Microsoft accounts, and made it very easy to switch from one to the other.