Description

We need to formalize/revise the current deprecation policy, which doesn't exist in writing yet.

References:

• https://www.doctrine-project.org/policies/deprecation.html
• https://www.drupal.org/about/core/policies/core-change-policies/how-to-deprecate
• https://discourse.laminas.dev/t/deprecation-policy-trigger-error-vs-deprecated/2435
• https://docs.djangoproject.com/en/6.0/internals/release-process/#deprecation-policy
• https://symfony.com/doc/current/contributing/code/conventions.html#deprecating-code