GMT 6.6.0 was released a few hours ago, and GMT 6.4 was released in June, 2022.

Following our support policy,

Drop support for GMT versions 3 years after their initial release, while ensuring at least two latest minor versions remain supported.

I think it's time to drop the GMT 6.4.0 support.

• GMT Legacy Tests: Bump from 6.4.0 to 6.5.0 Drop GMT 6.4 support  #4127
• GMT Tests: Bump from 6.5.0 to 6.6.0 Bump the GMT version to 6.6.0 in CI #4018
• Remove tests/codes related to GMT 6.4.0 Remove deprecated codes after dropping GMT 6.4 support #4128
• Bump minimum support GMT version to 6.5.0 Drop GMT 6.4 support  #4127
• Update the issue template bump_gmt_checklist.md Update issue template for bumping GMT versions #4019
• Unpin ghostscript in ci_tests_legacy.yaml Drop GMT 6.4 support  #4127