Short version:
The Microsoft Immersive Reader, which is currently built into Word, OneNote, Outlook, Teams and other Microsoft apps, will soon be available for SharePoint pages and news posts. Rollout begins in mid-March (Targeted release) and ends with Standard release (most of us) in mid-June 2021.

Details:
The Immersive Reader uses proven techniques to improve reading and writing for people regardless of their age or ability. This feature will be available on all modern pages and news posts except for home pages.

When you view SharePoint pages and news posts you will see a button on the page command bar to open the Immersive Reader. In this initial release, the Immersive Reader will only read content contained in text web parts.

Availability:
Rollout begins with Targeted release (select users and organisations) in mid-March and ends with Standard (most of us) release in mid-June 2021.

Links:
Learn more: All about the Immersive Reader – Microsoft Educator Center