We plan to publish a KB article on this soon. For now, the information you are looking for is somewhat scattered in the release notes for major releases.
This should provide the information you need:
Rebex R5 releases (version 5.0.7027 and higher)
Spport for these ended on 2023-10-10 (except .NET Compact Framework).
The latest release in these series is R5.14 (v5.0.8708) published on 2023-11-03.
Rebex R6 releases (version 6.0.8000 and higher)
These will be supported until at least 2024-10-10 2025-03-03 (or until Rebex R8 is published).
The most up-to-date release is R6.16 (v6.0.8930) published on 2024-06-12.
There will be at least one more R6.x release.
Rebex R7 releases (version 7.0.8581 and higher)
These will be supported until 2025-10-10.
The most up-to-date release is 7.0.8943) published on 2024-06-25.
Rebex R8 releases (versions 8.0.x)
These will become available later this year and will be supported until 2030-01-01.
If you are currently using R6.3 (v6.0.8123), you can easily upgrade to R6.16 (v6.0.8930) if you renew your support contract. Later, you can either upgrade to R7, or skip these series and upgrade to R8 when it's published.
Update 2024-09-09: Support date for R6.x was extended until at least 2025-03-03.