That is bashate, not ShellCheck, but there is good reason to use [[ since these operators are not POSIX to begin with and don't work in [.

Because the =~ and > and < operators aren't defined for [ in POSIX.

Also ShellCheck suggests similar improvement only when your shebang says that bash is the expected interpreter instead of sh (or other ways of denoting POSIX compat).

because its 2021 and we should get over compatibility with archaic tech?

In Debian/Ubuntu /bin/sh, most certainly.