How to retire old custom forms?

Based on your requirement you can choose from the below options:

Option 1: Hide the form from data entry

You can use this option when the users will not be entering for any of the future records.

You can use the JSON rules to hide a form and restrict all new data entry.
For steps please refer to Show/Hide Custom Forms based on Rules

Pros: You can stop all new data entry into the form.

Cons:

It will also hide existing entries from the overview page.
The user will have to run queries to see all historical records.

Option 2: Replace the old form with a new form

Use this option when the form is still active but needs updated fields or structure. This allows users to retain historical data and simplify ongoing data entry.

Export records from the old form using Export Form Records.
Create a new form with updated fields.
Modify the exported file so headers match the new form’s fields.
Import the data into the new form.
Delete the old form’s associations.

Pros:

Existing data continues to be accessible on the forms user interface.
Users are able to transition seamlessly to the updated version of the form.

Option 3: Direct Deletion

If you no longer wish to collect data using those forms, you can directly delete them.

Navigate to the forms module.
Select the form and delete it.
Note that this will remove all existing records tied to that form.