Programatically remove field content from all nodes where content equals 'string'?
The video_embed_field module adds a simple field based on whatever you named it.  So if your field is named "field_youtube_embed_test", you'd get a field on your content type that is a standard drupal field, with a table "node__field_youtube_embed_test" and a revision table "node_revision__field_youtube_embed_test".  You can run a query to delete all entries based on bundle as that is one of the columns on both tables.  So you can run a query for each.

DELETE from node__field_youtube_embed_test where field_youtube_embed_test_value = 'youtube.com/myPlaceholderVideo' and bundle = 'article';

DELETE from node_revision__field_video_embed where field_youtube_embed_test_value = 'youtube.com/myPlaceholderVideo' and bundle = 'article';


Or if you want to delete everything from those tables, you just use the truncate command:

TRUNCATE node__field_youtube_embed_test;

TRUNCATE node_revision__field_video_embed;


Using a query works for simple fields such as textfields, booleans, etc, but it doesn't work for more complex structures such as field collections or paragraphs.  For those, you would probably want to iterate through them programmatically and unset the value and then save the node.