• In general, connection closed from server is dangerous and user should avoid it by using SetConnMaxLifetime (and SetConnMaxIdleTime if needed). CheckConnLiveness reduces the risk but doesn't make it zero.

• CheckConnLiveness has performance penalty. (conncheck should be optional #996 )

• CheckConnLiveness makes difficult to read an unexpected packet from server. ("closing bad idle connection: unexpected read from socket" errors on MySQL 8.0.24 #1392 )

If Go supports nonblocking readable test, we can fix #996 and #1392.
Until then, how about disable the option by default in v1.8?
I don't talking about removing it so Githubber can keep using the option.