As of our release schedule the mentioned versions will go into security-only support mode. We will do a last round of bugfix releases together with the upcoming release of 7.1 to have a smooth transition with the new version available.
Afterwards those versions will still receive security fixes for one more year and run into EOL by April 2022.
Be sure to upgrade your projects to at least “March Hare” within this timeframe!
For new (non-security) pull requests created the lowest version to target should be 6.3 in the flow-development-collection and 5.3 in the neos-development-collection.