100% open source linux distro

To be clear, I'm using criteria that are usually termed as Free Software Guidelines. The two major versions of this that exist, which are very similar to be the point of being virtually indistinguishable, are the Debian and the FSF definitions.

Here is the FSF take on this. There is room for disagreement. Personally, Debian is plenty free enough for me.

Here is the FSF take on why certain distributions are not free, including Debian. At this point, the FSF problem with Debian is that it mentions/suggests non-free software, for example during the installation process. Also, it includes a non-free archive of software.

In any case, by the terms of the question, Debian should be free enough. You don't have to use the non-free components - they are clearly separated from the main (free) software archive.

Fedora has among its guidelines that nothing that can't be redistributed in source without legal restrictions (in the US, mainly because it is an US based distribution) is allowed. The exceptions to this are firmware for devices that the vendor allows to distribute freely, mostly distributed by/with the kernel. This is stuff that doesn't run on your principal CPU at all, it is taken as part of the device that is loaded at runtime.

Most Linux distributions have similar rules, partly for legal reasons. Ubuntu does distribute stuff that is covered by patents in the US, as it lives in another jurisdiction they can get away with it (but in principle the user could be sued over it...). And some time back there were import/export troubles to/from the US with a range of cryptography software, forcing different packages depending on your residence.