Just to note a small nuance on what Sander said.
If, following code-freeze, you suddenly find an improvement / new feature that you think has a really good case to still be included, then there is a process involving the 'unhold_requested' tag in the tracker which you can use if you really think it is important enough: https://docs.moodle.org/dev/Integration_Review#During_continuous_integration.2FFreeze.2FQA_period. Not to be abused!
If, following code-freeze, you suddenly find an improvement / new feature that you think has a really good case to still be included, then there is a process involving the 'unhold_requested' tag in the tracker which you can use if you really think it is important enough: https://docs.moodle.org/dev/Integration_Review#During_continuous_integration.2FFreeze.2FQA_period. Not to be abused!