One can change online and start article properties of a frozen article.
In properties tab editing these options is available but a frozen article should not be changed. Disable these options.
Code: Make sure changes from properties page are not saved if they are submitted.
GUI: Disable options to set an article offline or as start article in properties tab and in overview tab. Add a description why the option is not available to avoid end user confusion.
Steps to reproduce:
Go to Content -> Articles
Click on a category
Freeze an article
Go to properties tab of frozen article