Add a possibility for exclude paragraph types

Not sure what you want to do exactly, but did you try with disabling all, as the help text says, "All disabled is all enabled".

1. +++ b/src/Plugin/EntityReferenceSelection/ParagraphSelection.php
   @@ -50,6 +51,15 @@ class ParagraphSelection extends DefaultSelection {
   +      '#title' => $this->t('Exclude paragraphs type'),
   

   Maybe "Choose allowed paragraph types" or "Choose paragraph types" would be more proper, since the exclude might mix with the include option and would not make sense.

2. +++ b/src/Plugin/EntityReferenceSelection/ParagraphSelection.php
   @@ -50,6 +51,15 @@ class ParagraphSelection extends DefaultSelection {
   +    $form['negotate'] = array(
   

   array() -> []

3. Needs test coverage.

Wrote a test for this issue.

Added the negotate item to the schema + made some changes to the backend screen. I changed the select to a checkbox. What do you think?

Please always add screenshots for UI change proposals. It allows much easier UX review.

Undid the UI changes.

Before adding more configurability to Paragraphs that is not simply catching up with core patterns, we should define the final goal of how we want to manage paragraph types.

That makes it related to the paragraph type grouping we are also introducing:
#2831762: Add grouping for Paragraph types in "Modal" add mode

People asked to define sets of paragraph types and only allow that set, so adding a new type to a set doesn't require editing every single other paragraph type adding that new type... Others are proposing better tools for batch updates.
See also #2504583: A way to select between predefined sets of paragraphs?

Added in the experimental and change some descriptions.

Screenshot:

Renamed config setting negotate -> negate

1. +++ b/src/Plugin/EntityReferenceSelection/ParagraphSelection.php
   @@ -154,4 +164,77 @@ class ParagraphSelection extends DefaultSelection {
   +        $query->condition($entity_type->getKey('id'), NULL, '=');
   

   I don't think this is correct.
   (BTW some more comments in the logic would help read it faster.)

   If no paragraph type is selected, thus it seems getAllowedTypes() returns emptyness now, still all should be allowed.

   Is this selection thing well test covered or am i wrong?

2. +++ b/src/Plugin/Field/FieldWidget/InlineParagraphsWidget.php
   @@ -707,6 +707,10 @@ class InlineParagraphsWidget extends WidgetBase {
   +      if ($this->getSelectionHandlerSetting('negate')) {
   +        $bundles = \Drupal::service('entity_type.bundle.info')->getBundleInfo($target_type);
   +        $bundles = array_diff_key($bundles, $this->getSelectionHandlerSetting('target_bundles'));
   
   +++ b/src/Plugin/Field/FieldWidget/ParagraphsWidget.php
   @@ -684,6 +684,10 @@ class ParagraphsWidget extends WidgetBase {
   +      if ($this->getSelectionHandlerSetting('negate')) {
   +        $bundles = \Drupal::service('entity_type.bundle.info')->getBundleInfo($target_type);
   +        $bundles = array_diff_key($bundles, $this->getSelectionHandlerSetting('target_bundles'));
   

   I'm very unhappy we need to put this type of setting into the widget.

   Instead, a heper method should just interprete the setting and provide us the list of allowed paragraph types. Like getAllowedBundles().

Created an issue #2881145: Get allowed paragraph types from ParagraphSelection, this can wait for that

This is build on top of #2881145: Get allowed paragraph types from ParagraphSelection

#2881145: Get allowed paragraph types from ParagraphSelection is committed.
So adding a patch with the negate setting. Works properly atm.
Screenshot of the form:

1. +++ b/config/schema/paragraphs_type.schema.yml
   @@ -37,6 +37,8 @@ paragraphs.behavior.settings.*:
   +      type: boolean
   

   Wouldn't this be integer? Because I think if its bool, radios would interprete false as not selected.

2. +++ b/src/Plugin/EntityReferenceSelection/ParagraphSelection.php
   @@ -50,6 +51,18 @@ class ParagraphSelection extends DefaultSelection {
   +      '#options' => [
   +        1 => 'All except those selected',
   +        0 => 'None except those selected'
   

   Options are not translated.

   Also I am not sure about those options. Maybe like that (not really happy with those either)?
   1 All selected below
   0 All, except the ones selected below

3. +++ b/src/Plugin/EntityReferenceSelection/ParagraphSelection.php
   @@ -50,6 +51,18 @@ class ParagraphSelection extends DefaultSelection {
   +      '#return_value' => 1,
   

   I am not really familiar with return_value. What does this do?

4. +++ b/src/Plugin/EntityReferenceSelection/ParagraphSelection.php
   @@ -50,6 +51,18 @@ class ParagraphSelection extends DefaultSelection {
   +      '#default_value' => isset($selection_handler_settings['negate']) ? $selection_handler_settings['negate'] : FALSE,
   

   Hm, if we change the schema to int, we might want to fallback to 0 here.

5. +++ b/src/Plugin/EntityReferenceSelection/ParagraphSelection.php
   @@ -168,7 +181,12 @@ class ParagraphSelection extends DefaultSelection {
   -      $bundles = array_intersect_key($bundles, $this->configuration['handler_settings']['target_bundles']);
   +      if (isset($this->configuration['handler_settings']['negate']) && $this->configuration['handler_settings']['negate'] == '1') {
   +        $bundles = array_diff_key($bundles, $this->configuration['handler_settings']['target_bundles']);
   

   So if we make this a int, will it still evaluate to '1'?

Addressed #37.
For the descriptions I used:
Exclude the selected below
Include the selected below

Committed. :-)

But i noticed that the base of ParagraphSelection is DefaultSelection that implements many methods that are not overridden. There are methods like getReferenceableEntities() countReferenceableEntities() buildEntityQuery() and even createNewEntity() that keep the old (at least not negated) behavior. Where are those methods used?
Needs work for discussion and then to be closed.

Yes, this was too early, it's has incomplete test coverage. Validation doesn't work and entities can't be saved.

Not sure if it's worth to revert, @Ginovski is working on improving it.

Adding buildEntityQuery() and validate.

Fixed methods and extended tests.

Adding test-only patch.

Committed, thx. :-)