Depends on D20666. Ref T13343. In D20666, I limited the rate at which a given user account can be sent account recovery links.

Here, add a companion limit to the rate at which a given remote address may request recovery of any account. This limit is a little more forgiving since reasonable users may plausibly try multiple variations of several email addresses, make typos, etc. The goal is just to hinder attackers from fishing for every address under the sun on installs with no CAPTCHA configured and no broad-spectrum VPN-style access controls.