As reported by s.kavita in MDLQA-11701:

Steps to reproduce:

1. Create a forum and set the subscription mode to disabled.
2. Log in as a student and add a new discussion topic.
3. In the form, mouse over the discussion subscription checkbox.

Expected result: A disabled checkbox icon is displayed.

Actual result: No icon is displayed.

Note: This bug is only present when using Boost in 3.5. Clean is not affected nor is Boost in 3.4.2.