strtolower is deprecated

I don't think strtolower itself is deprecated: https://www.php.net/manual/en/function.strtolower.php

Did you recently update to the 2.0 release? Did you run update.php? (drush updb)

Added a few checks to really make sure there's a label string.

This works for me.

Starting with a vanilla Drupal install (I already have layout_builder enabled and a 'test' page; any content type with layout_builder enabled for it).
Delete the template and section label config settings.
drush config:delete section_library.settings template_label
drush config:delete section_library.settings section_label
Either go to a page with an existing layout or create a new one, then go to the layout tab; you should see 2 error messages ().

Apply the patch and either go back the page you just used or create a new one; you'll no longer see the error messages :)