9.4 - Custom Fields ID auto-increment might causes broken email placeholders#4902

Hello,

I would like to report an issue that initially appears as an ID handling problem in Custom Fields, but actually leads to broken functionality in email templates.

Initial issue:

• When deleting a custom field and creating a new one, the ID continues incrementing instead of reusing available IDs.
• IDs are not editable and remain exposed in UI.

Deeper issue (functional impact):

• Email template placeholders depend on these IDs (e.g. {custom_field_7})
• When a field is deleted, the placeholder remains in templates
• A newly created field receives a different ID (e.g. {custom_field_10})
• Result: placeholders break and emails no longer contain expected data

Why this is a problem:

• IDs are treated as stable references, but they are not stable
• No validation or warning exists when deleting fields used in templates
• No automatic remapping or fallback mechanism

Conclusion:
The ID auto-increment behavior itself is not the core issue,
but combined with placeholder dependency, it creates a reliability problem.

Expected improvements:

• Use stable identifiers (slug/UUID) instead of numeric IDs in placeholders
  OR
• Prevent deletion of fields used in templates (or warn user)
  OR
• Provide automatic placeholder remapping

This issue affects production reliability, not just UI behavior.

Thank you.