I strongly recommend that this feature be re-added before the "update" goes fully live. The ability to search across multiple email accounts is the main reason for using Outlook desktop, it is why we don't use the online version of Outlook.
In our company, we have multiple teams that may touch an order (customer service, purchasing, tracking, etc.). It is very important to have the ability to search all accounts for on order and see everything / everyone who may have had been involved so that we have a full picture of any issues.
For many of our computers, we specifically purchased the desktop licenses simply so they could use the Outlook desktop app to search multiple email accounts at once. Remove this feature, and we will no longer have a reason to pay for the desktop license for many of our team members. If this function did not exists, we would probably purchase a third party software that had this function, to manage our emails.