This page of the documentation:
suggests that there is supposed to be an option to generate a Safari Extension Certificate somewhere on the "Certificates, IDs & Profiles" page. However I don't see the word "Safari" or "extension" anywhere in this section. How can I generate a Safari Extension Certificate?
You're after a 'developer certificate', not a Safari Developer certificate/Safari Extension Certificate
From the SEGD - Using Extension Builder:
"Before you can build and install an extension, you need to install a developer certificate. For more information, see Developer Certificates."
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>How can I generate
See: Your Signing Certificates in Depth
Question - are you working with a Safari extension or Safari app extension? Because, as the docs note 'you don’t use Safari Extension Builder to create an app extension. Instead, you have an Xcode project.'
How you deal with that certificate might vary, depending on. I'm assuming you're working w/Xcode.
Dont use Safari, it is deprecated. You can not distribute new extentions , only update existing ones with this certification. Safari Developer Certification could be find at bottom in certification page under Legacy section.Good luck
Note you're making recommendations on 2 year old conditions...
I am not going to be rude on here but when the root certificate is expired we can just create a fake developer cert by law because even using the official certificate service would require a repair of the planned obsolescence. Contact me if you would like a like to some tutorials. Apple, you really should allow us to sign a certificate for old computers for free since it is of no material extra cost to do and because if you are caught in federal court using certs to blacklist it could be bad.