after unlocking ostree, why the repository can’t fetch updates from Endless repo?
Unlocking the ostree is unsupported, but OK for experimentation if you’re happy to deal with any irreversible breakage. However it’s not possible to advise further if you don’t provide more details (what commands did you use, what results did you expect, what results did you observe, which OS version are you running, etc).
after unlocked, add Debian/ubuntu repository work but I’m afraid that will break all the EOS packaging dependencies
Yes, deviation away from Endless’s debian base is likely to be problematic, but you’ll likely be OK if you stick to the same Debian base (e.g. you can use Debian bullseye with eos4 likely without too many difficulties; eos-enable-debian-apt is another unsupported-but-potentially-useful tool)
dpkg-deb: error: archive ‘v4l2loopback-utils_0.12.5-1ubuntu3_all.deb’ uses unknown compression for member ‘control.tar.zst’, giving up
Looks like the package you are using is not compatible with the version of apt that is present on your installation. I imagine the same would happen if you took an old version of Ubuntu and tried to use brand new packages like this.
Our purpose of unlocking the ostree is to try out the printer or scanner that have released a driver for debian/ubuntu and some other thing like the v4l2loopback for webcam.
Which printer/scanner (& associated driver) are you trying to test here? If there is support in official Debian/Ubuntu then we may be able to include it in official releases. Your idea to use Debian packages as a one-off experiment is sensible but only if you align the Debian version used by Endless with the Debian version that the package is built for.
Building external kernel modules such as v4l2loopback is likely much harder; feel free to try but I would expect this to bring up several challenges on the way.