Microsoft will be removing the ability to add new accounts to Outlook on the web using the Connected accounts feature in September 2018.
When will this feature be removed?
You won’t be able to connect new accounts after September 15, 2018.
What will happen to the accounts that I am already connected to?
Accounts that were connected before September 15th will continue to sync as usual until October 30, 2018. On October 30th, all connected accounts will stop syncing email.
When will the Connected accounts option be removed?
On November 15th, the Connected accounts option (Settings > Mail > Accounts) will be removed from Outlook on the web .
What will happen to the emails that have been already synced to my account?
Email that has already synced will continue to appear in your inbox unless you remove it manually.
Are there more details regarding this change?
Yes, please review Microsoft's release on this change.