Microsoft is working on adding support to the Outlook on the web browser-based client for sending emails via alias email addresses (also known as aliases or proxy addresses) Microsoft is also planning to add protection against Reply-All email storms in Exchange Online sometime during Q3 2020. Microsoft is currently enhancing the way emails sent using the Office 365 Message Encryption (OME) service are seen by mail servers with the end goal of making them less likely to be marked as spam and automatically sent to the Trash folder.
Source: https://www.bleepingcomputer.com/news/microsoft/microsoft-outlook-for-the-web-to-support-sending-email-as-alias/