Twig template not rendering new field variable
@4k4, thanks for sharing the link. I was able to retrieve the value
this way: content.field_hide_header_title[0]

content.field_myfield[0] still gets the rendered output, now of the first field item. It's not a good idea to use it for programming, because the output can be formatted in different ways and is translatable. You don't want to adjust the code anytime this is changing.
For programming if/else code better use the block content object.
content['#block_content'].field_myfield.value
This is a similar question for a node field: Boolean field inside node twig renders ok but does not work in if statement