Microsoft intends to add the @Everyone mention to allow users to notify everyone in group and meeting chats about a message, following the feature that lets users add people to Teams group chats with a @mention (a feature that was supposed to debut in late December but appears to have been delayed).
Microsoft anticipates rolling out the functionality in late January 2023, with completion slated for late February, according to message center notice MC485628 (15 December 2022, Microsoft 365 roadmap item 101573). The function will be compatible with PC, iOS, and Android users. Browser clients are not mentioned.