When did WSL's --import add support for xz and zstd?

The conclusion is: I could not find an official release note that says "supported from this version onward." However, there are lower bounds that can be verified from publicly available information.

Format	Earliest verifiable evidence from public sources	Confidence
.tar.xz	WSL 1.1.3.0, as of 2023-03-12	Relatively high
.tar.zst	WSL 2.1.5, as of 2024-04-11	Moderate

.tar.xz Was Working at Least in WSL 1.1.3.0

In GitHub issue #9778, the reported environment is WSL Version 1.1.3.0, and the reproduction steps include:

wsl --import ... lunar-server-cloudimg-amd64-root.tar.xz

The issue itself is not about import failures but about problems opening the imported distribution from VS Code afterward. That suggests that using .tar.xz with wsl --import was already a valid workflow at that time. (GitHub)

.tar.zst Was Working at Least in WSL 2.1.5

In a comment on GitHub issue #6056, dated 2024-04-11, a user reported that on WSL 2.1.5 (latest at the time), importing .tar.gz, .tar.xz, and .tar.zst all worked.

This is a community report rather than an official release note, but it is still useful as the earliest confirmation I could find for .tar.zst support. (GitHub)

As of 2020, Support Was Not Yet an Explicitly Supported Feature

Issue #6056, opened in October 2020, was a feature request asking for .xz support in wsl.exe --import and the WSL Distro Launcher ecosystem.

The issue notes that the expected formats at the time were install.tar and gzip-compressed archives such as install.tar.gz or install.tgz, and requests support for XZ-compressed archives.

In other words, as of 2020, .tar.xz was at least not an explicitly documented or officially assumed format. (GitHub)

Internally, Import Does Not Appear to Hard-Code Compression Formats

Looking at the current WSL source code, the wsl --import path appears to reject .vhd and .vhdx when passed without --vhd, then opens the file and passes it to RegisterDistribution.

The wsl.exe client code does not appear to explicitly branch on .gz, .xz, or .zst extensions. (GitHub)

By contrast, the --export side exposes explicit format options such as:

• tar.gz
• tar.xz
• tar
• vhd

This suggests that export explicitly enumerates output formats, while import largely hands a tar-based archive to backend extraction logic and lets that layer handle the compression format. (GitHub)

Summary

Strictly speaking:

.tar.xz was being used with wsl --import no later than WSL 1.1.3.0 (March 2023).

.tar.zst has confirmed working reports no later than WSL 2.1.5 (April 2024).

However, no official changelog has been found that explicitly states when support for either format was first introduced.

Therefore, support did not first appear in WSL 2.5.x. In particular, .tar.xz appears to have been working significantly earlier.