Thanks Google :( I think they should at least put a rate limiter to that endpoint.
1. Checking if an email address exists
2. Running it against a known dump of leaked data, with passwords etc
3. Try logging in to google account with the leaked password, hoping the user reuses passwords
Google encourages their users to use 2FA and has other measures to detect when logins are coming from unknown locations, so I guess they figured the risk of this was pretty low
Everybody should enable 2FA, and use the strongest 2FA you can. Buy a yubikey or other U2F key and use it for everything possible. And webdevs, please start supporting U2F in addition to RFC 6238 TOTPs. It's really not that hard.