I can't change permissions for text filters
This is a usability quirk of D8, in the Drupal Form UI the default format type for formatted text fields isn't configurable; it's dynamic based off form element properties (set by a developer), user role permissions, and the filter module settings (see form code)
By default, the fallback format (Plain text here) won't show as an option. There are a few things you can do here:

Set Plain text as the highest priority filter, if you want it to be the default for all formatted text fields.
Enable the always_show_fallback_choice config option (e.g. drush config-set filter.settings always_show_fallback_choice true), if you just want the format to be available.
Use the Allowed Formats module, if you need some type of per-field configuration.