Is there a Safari Extension to rename email subjects similar to what's available in Chrome?

Correct dehopley,

There is no Apple app that will do what you're wanting to do. Also, when we searched in the Mac App Store with the search parameters of "Safari change Gmail email subject", no apps came up there either.

Take care.

Hi there dehopley,

To see extensions available in Safari, follow the steps below:

1. In the Safari app  on your Mac, choose Safari > Safari Extensions, then browse the available extensions.

You'll find this information here: Get extensions to customize Safari on Mac - Apple Support

If you don't see an extension that will accomplish what you'd like, you can provide feedback here: Feedback - Safari - Apple

Take care.