The method `add_template_helper` does not seem to work in Rails 6.1

Question

After updating my project to Rails 6.1 I get this error:

NoMethodError:
  undefined method `add_template_helper' for ApplicationMailer:Class

In the documentation does not seem to be deprecated.

user_2995613 · Answer

So, apparently, this method was deleted without deprecation warning in this commit:

So, the fix for this issue is to replace:

class MyMailer < ApplicationMailer
  add_template_helper MyHelper

With:

class MyMailer < ApplicationMailer
  helper MyHelper