Why doesn't Microsoft Outlook allow formatting text in the subject line?

Answer 1

This is by design. Microsoft does not allow for subject line editing so that when a message is sent via e-mail, the user can understand in plain text what the context of an e-mail is. Outlook was designed for businesses and not for personal use initially. Businesses did not use emojis or other formatting factors when sending a message.

Because Subject text cannot be modified, there is no worry of a misunderstanding or a spam filter marking an e-mail as spam because there is a character in the subject that the filter views as flagged.