Supporting several release lines: which fixes go where

What it is

A supported-versions policy answers "will version A.B get this fix?" before anyone asks. It has two parts: a table of release lines with their status and dates, and backport rules per fix class. The Django release process is a compact example: the main branch gets features; the last feature release gets fixes for release blockers (security, data loss, crashes, major bugs in new features, regressions); security and data-loss fixes go to main, the last two feature branches and any supported long-term-support branch; documentation fixes are backported more freely. PEP 602 describes the Python series as maintained for five years: bugfix releases for the first two, then security-only source releases for three more (from Python 3.13 onward). The Rust book states the opposite extreme: only the most recent stable version is supported, so each version is supported for six weeks.

Why it matters

Users cannot upgrade on the project's schedule, and a maintainer cannot fix every line forever. Without a written rule, each request for a backport becomes a negotiation, and security fixes silently land only on main while users of the previous line believe they are covered.

How to apply

• Publish the table where users look: line, status, first release, planned end of life. Keep one branch per supported line, named consistently (Django uses stable/A.B.x).
• Default for a small project: the latest feature release gets all fixes; the previous one gets security fixes for a stated period; nothing else. Widen only when a second maintainer exists.
• Backport by cherry-pick with the original commit reference in the message, and release the backport; a fix on a branch without a release helps nobody.
• Restrict the CI matrix to supported lines; an unsupported line that is still tested invites requests to fix it.
• Announce end of life in advance in the changelog and remove the line from the table on the date.
• Offer long-term support only if the years can actually be promised; an LTS label that is abandoned is worse than none.

Pitfalls

Backporting features under the name of fixes, which turns a stable line into a second development line. Fixing a security issue on main only. Letting the CI matrix grow with every supported version of every dependency until nothing is green. Forgetting that documentation for old lines also needs to stay reachable.