This was announced in a letter sent to G Suite users. The reason is the high vulnerability to account hijacking when using single-factor authentication by login and password.
On June 15, 2020, the possibility of password authentication will be disabled for users who are trying to implement it for the first time, and on February 15, 2021 - for everyone.
As a replacement, it is suggested to use OAuth. Of the free clients for IMAP, CardDAV and CalDAV, Thunderbird and KMail support this authentication method (but KMail users have recently noticed problems).
Password authentication for SMTP will continue to work. Similar changes for non-corporate users of Google accounts are not yet known.
Source: linux.org.ru