Unexpected register_rest_route messages

[bobbingwide]

<b>Notice</b>:  register_rest_route was called <strong>incorrectly</strong>. 
The REST API route definition for <code>udemy/v1/login</code> is missing the required <code>permission_callback</code> argument. 
For REST API routes that are intended to be public, use <code>__return_true</code> as the permission callback. 
Please see <a href="https://wordpress.org/support/article/debugging-in-wordpress/">Debugging in WordPress</a> for more information. 
(This message was added in version 5.5.0.) in <b>C:\apache\htdocs\wp56\wp-includes\functions.php</b> on line <b>5225</b><br />

plus 4 or 5 more.

[bobbingwide]

These messages were coming from some mu-plugin files that I'd put into the local environment after a talk at WordPress Brighton back in May. Nothing to do with Gutenberg.