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The problem with this is that it is immediately obvious to anyone who knows this how to go from john+that_scammy_site@example.comto john@example.com

It's not the same though. Fastmail allows a@fastmail.com and b@fastmail.com going to the same inbox. It's also easy to create these aliases.

You have revealed your real email using the gmail plus sign feature. It's not that difficult for the people selling the emails or the spammers to remove a plus sign and some characters after years of knowing this.

What prevents spammers from recognizing the + pattern and omit it?

Most of the time they don't care. They've got 4 million email addresses, they'll just spam all of them.

I've been using this for years. IIRC it's called "chaining". Unfortunately I've come across many sites that don't recognize, or allow, the + character in their input field. For those sites i can still crates either a group email from my Gmail, or just add another alias.