The runtimes are not locked down, anyone can publish theirs (under their domain name, ofcourse), so it depends on who publishes it.

For the freedesktop one, they have following policy[1]:

  - When a new stable release release is done, the changes on that branch will only be:
    - security updates
    - stable releases (tested carefully); no ABI breaks / API build breaks.
    - we will try to keep updating that branch as much as we can

  - We release a new major release every year, only if:
    - There is a ABI break
    - There is a "API build" break (apps might not compile because new major releases of important packages (GCC))
    - Looking at the GCC and other major project cadences, this is likely going to happen annually.

  - We only maintain the current stable release and the previous one, this means:
    - Stable releases get 2 years of security updates
    - We maintain maximum 3 releases at any given time:
      - Development
      - Stable
      - Old stable

[1] https://gitlab.com/freedesktop-sdk/freedesktop-sdk/wikis/rel...