Hacker News Reader

I assume spammers have become adept at stripping anything between + and @ in email addresses, so this isn't as reliable as properly masked addresses.

That's been my experience too. That's why prefix inboxes allow any suffix, including "-", "_", "$", "!" etc.

The “x+tag@domain.com” format is also rejected by many validators, sadly. I thought this was a gmail feature, but was corrected[0][1] a while ago.