EndeavourOS is based on Arch. Arch Linux, as of now doesn’t separate free and non-free packages. Some non free software will be found in the official Arch repos.
On EnOS, I think you’ll have to manually check the license of packages, which can get difficult if a package pulls in many dependencies. I hope someone else is able to help you with your query.
There is Parabola disro, which is Arch, but only free software.
there are some stuff with closed sources (propritary) installed for EndeavourOS for sure, like if you choose Nvidia driver to be installed plus common libs for audio and video usage (mp3 e.t.c.) but we do not have an option to not install them per default, you could only remove them from beeing installed with netinstall, or remove them after installing the system.
As already mentioned I would suggest using parabola. I don’t know about your hardware specifically but as Joe mentioned you do have NVIDIA drivers your not going to have a very good day trying to only run open source. If your values are that strong and you do have Nvidia you may need to consider purchasing other equipments.