Placing the following line inside the source of a node's body field adds a manual teaser break:
<!--break-->
This works fine when the node uses the "Full HTML" text format. It stops working, however, when I switch the text format to "Basic HTML". I expect manual teaser breaks to work with any text format, if not out of the box then at least by configuring the text format in some way (e.g. by modifying the text format's set of allowed HTML tags).
A similar issue already exists for Drupal 7, but In Drupal 7 a workaround exists because it's possible to add <!--> to the "Basic HTML" text format's set of allowed HTML tags (cf. comment 118 of the other issue).
The same workaround does not work for Drupal 10 because in Drupal 10 "Basic HTML" treats the set of allowed HTML tags as DOM elements (cf. implementation of the function FilterHtml::getHtmlRestrictions()), but the manual teaser break is a HTML comment.