I think it would be important to archive Libreboot releases.
The issue is that while it is possible to suggest to archive specific git repositories through the web interface, as I understand, it's not possible to do it for files in various format (tarballs, README files, etc). Though GNU releases are somehow archived.
The official URL for the Libreboot releases is https://rsync.libreboot.org/, and the files can be downloaded through rsync or https.
Not all the files inside are source code though. This probably complicates the archiving.
Here's an analysis of the files found:
|ccsource||Trisquel source code, can probably be skipped|
|oldstable||oldstable/README.txt oldstable/*/*docs* oldstable/*/*source* oldstable/*/*src* oldstable/*/*sum* oldstable/*/*SUM* can be archived|
|stable||stable/README.txt stable/*/*lbwww* stable/*/*src* stable/*/*sha* stable/*/*SUM* can be archived|
|testing||testing/README.txt and testing/*/*src* can be archived|
|u-boot-libre||See below, all files can probably be archived|
|unstable||Contains a README.txt file that can be archived|
The top files lbkey.asc and lbkeyold.asc can also be archived.
Libreboot releases proper
It consists of files (including tarballs) but as I understand gnu.org is similar and it's archived. It should normally be 100% free software (with various free licenses).
In the table before I've added python glob compatible regex to find the interesting files. It picks a bit more checksums than necessary though.
Files under u-boot-libre/
Files in in https://rsync.libreboot.org/u-boot-libre are a bit special: they are releases for a 100% free software version of u-boot in the same spirit than linux-libre.
So this directory only contains source code, checksums and signatures under various formats.
So if new nonfree files are found in u-boot, the old releases will likely be removed and replaced by newer ones without the newly found nonfree files.
So far the only problematic found in the current u-boot-libre archives is drivers/dma/MCD_tasks.c which contains a firmware under GPLv2+ but without the corresponding source code.
Being able to archive Libreboot source files could enable various GNU/Linux distributions to continue to use them if for some reasons the libreboot websites goes down.
Parabola (an FSF approved GNU/Linux distribution) already makes use of both Libreboot source code (https://git.parabola.nu/abslibre/abslibre.git/tree/pcr/libreboot-utils/PKGBUILD) and u-boot-libre source code (https://git.parabola.nu/abslibre/abslibre.git/tree/libre/uboot4extlinux-sunxi/PKGBUILD).