Undo file format version 3 in Vim 9.0 blocks cross-version undofile sharing

Goal

Ensure that a repeatable development environment can persist undo history across different Vim versions used by team members.

Constraint

Vim 9.0 writes undo files using format version 3, which Vim 8.2 and earlier cannot read, leading to silent loss of undo history when the same undofile is accessed with an older binary.

Uncertainty

It is unclear whether Vim should provide a backward-compatibility mode or a version-selector option that forces the legacy undo file format by default, or if users must manage this externally.

Should Vim introduce a configuration option to write undo files in the legacy format?

Can an automatic detection mechanism be added to select the appropriate undo format based on the Vim version that will later read the file?

What impact would forcing the legacy format have on undo features introduced in Vim 9.0?