Block configuration entities need a region, but not blocks in general. Twig Tweak can generate block configurations on the fly and doesn't need them to be stored in the database:
{{ drupal_block('plugin_id', {label: 'Example'|t, some_setting: 'example', setting_array: {value: value}}) }}
https://git.drupalcode.org/project/twig_tweak/-/blob/3.x/docs/blocks.md#block-plugin