Upcoming PostgreSQL 14, supposed to be released by the end of 2021, will remove the support for version 2 frontend/backend protocol (in this article I refer it as "v2" or "v2 protocol").
What is version 2 frontend/backend protocol?
It's a one of communication protocols used between clients and PostgreSQL. For example, to send a query to PostgreSQL, clients first send a letter 'Q' then the actual SQL query string. The letter 'Q' indicates the client wants to send a query to PostgreSQL. In 2003 PostgreSQL 7.4 was released and started to support newer protocol v3. From PostgreSQL 7.4 to PostgreSQL 13, they support both older protocol version 2 and newer version of protocol v3. Since v3 protocol is better than v2, especially in that v3 supports extended query which allows to use prepared statements, the support for v2 is just for a backward compatibility sake. Since PostgreSQL 7.4 was released in 2003, PostgreSQL keeps the v2 support for 18 years!
Any trouble caused by dropping v2 protocol?
Good question. If you are using older drivers and the driver use v2 with your application, you may get into a trouble because PostgreSQL 14 (and newer) will not accept your application any more. For example, if you are using older version of JDBC driver (older than Version 9.4.1210 released in 2016-09-07, "fix: support cases when user-provided queries have 'returning' PR#488 c3d8571" is the commit message which drops the v2 support), the chances are some of the code of the application may choose v2 protocol. If you are using such older drivers, I recommend you to test to connect to PostgreSQL 14 using the application with the driver. As of today, PostgreSQL 14 beta1 is available.
What about Pgpool-II?
Pgpool-II supports both v2 and v3 protocol as of Pgpool-II 4.2. For upcoming Pgpool-II 4.3 which is supposed to be released by the end of 2021, developers have not decided yet the support for v2 will be dropped in 4.3 or not. Dropping v2 support code will remove non-negligible amount code from Pgpool-II and it makes developers' life a little bit easier. If nobody is using v2 protocol with Pgpool-II, we can happily remove the v2 code. If you are still using v2 protocol with Pgpool-II, please reply back to the mailing list thread.