Turn off the front end indicator - xwp/stream GitHub Wiki

By default, Stream displays a lightweight front-end indicator to show which version is installed and activated.

It's an HTML comment that looks like this:

<!-- Stream WordPress user activity plugin v2.0.0 -->

If you want to turn this off, there is a filter available for doing so:

add_filter( 'wp_stream_frontend_indicator', '__return_false' );

Read about __return_false in the WordPress Codex.