Installing AutoplayStopper stopped videos autoplaying for me and for others. YMMV.
For more than a year (installed 1 Sept 2019) I’ve been using AutoplayStopper. The extension does what it claims to do; others report similar satisfaction.
Mid-May 2020, a Brave moderator suggested Brave may have a fix in the works. Maybe. The Brave moderator then closed discussions of this issue in that thread.
The not-so-good news — resolving this (and other issues) with extensions has potential downsides: https://krebsonsecurity.com/2020/03/the-case-for-limiting-your-browser-extensions/