You can add strikethrough in email subject lines using Unicode rather than HTML. You can convert text to Unicode for formatting using this TextConverter. However, please note that Gmail may detect this as possible spam due to formatting inconsistencies.
Here is an example where strike through is used in the subject of an email campaign:
![Example of test email with strike through subject line.](https://mailbluster.com/wp-content/uploads/2024/07/Strike-through-in-Email.png)
And, here is the output of the email.
![Example of email with strike through subject line.](https://mailbluster.com/wp-content/uploads/2024/07/Strikethrough-in-Subject-line.png)