how to make method of class ComputedPhpFormatter?

Problem/Motivation

The option to enter PHP code via the Web UI was removed in Drupal 9+ for security reasons.
Then how to module method of class ComputedPhpFormatter ?
Eg. age into method of class ComputedPhpFormatter

Steps to reproduce

ComputedPhpFormatterExample.php

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes