Problem Definition:

Currently when an issue is excluded from a plan you can edit the issue sources under the plan settings "Configure >> Issue Sources >> Edit Issue Sources"  then the items that were excluded will show up unselected, and you can manually review these items to get a list of issues or releases excluded from a plan this way and you can re-add those items to to the plan by reselecting them and submitting the changes.

However, there are no other ways to generate a list of issues or releases excluded from a plan. If an issue or release is accidentally excluded there is no easy way to identify the item other than the method outlined above

Suggested Solution

Provide a report in portfolio with a list of issues OR a JQL search option such as:

issue IN plan()
issue NOT IN plan()
issue Excluded from plan()
fixVersion IN plan()
fixVersion NOT IN plan()
fixVersion Excluded from plan()

Any option to generate a full list of items that are expressly excluded from a plan for easy identification

Workaround

To look up version exclusions in the DB:

SELECT * FROM "AO_D9132D_EXCLUDED_VERSIONS";

Excluded issues are saved in the "entity_property" table noting the entity_id aligns with the issue id on the jiraissue table:

SELECT * FROM entity_property WHERE ENTITY_NAME='IssueProperty' AND PROPERTY_KEY='jpo-exclude-from-plan';