The message
Deprecated function: Creation of dynamic property H5peditor::$content is deprecated in H5peditor->processParameters() (line 158 of ./vendor/h5p/h5p-editor/h5peditor.class.php).
occurs when using h5p/h5p-editor:1.25.
The issue has been fixed in the master branch of https://github.com/h5p/h5p-editor-php-library but not yet in a tagged release. The composer.json in this module needs to have the version of the dependency set to "h5p/h5p-editor: dev-master".
Active
2.0
Code
I think we could potentially update to dev-master while the 2.0.x release is in alpha/beta status. But ideally stable releases of the Drupal H5P module would be able to rely on stable releases of the H5P libraries.
https://github.com/h5p/h5p-editor-php-library/issues/276 looks like the closest to an upstream issue asking for a new release, so linking to it here.