$ rpm-ostree rebase fedora:fedora/44/x86_64/silverblue2 metadata, 0 content objects fetched; 828 B transferred in 2 seconds; 0 bytes content writtenChecking out tree a04a55e… doneEnabled rpm-md repositories: fedora-cisco-openh264 updates fedora copr:copr.fedorainfracloud.org:phracek:PyCharm google-chrome rpmfusion-nonfree-steam rpmfusion-nonfree-nvidia-driver updates-archiveUpdating metadata for ‘rpmfusion-nonfree-steam’… doneerror: Updating rpm-md repo ‘rpmfusion-nonfree-steam’: Failed to download gpg key for repo ‘rpmfusion-nonfree-steam’: Curl error (37): Could not read a file:// file for file:///usr/share/distribution-gpg-keys/rpmfusion/RPM-GPG-KEY-rpmfusion-nonfree-fedora-44 [Couldn’t open file /usr/share/distribution-gpg-keys/rpmfusion/RPM-GPG-KEY-rpmfusion-nonfree-fedora-44]
Your output was unreadable; pre-formatted text </> retains formatting, and is considerably better than the mess posted above. I’ve stripped out the error message.
Checking out tree a04a55e… done
Enabled rpm-md repositories: fedora-cisco-openh264 updates fedora copr:copr.fedorainfracloud.org:phracek:PyCharm google-chrome rpmfusion-nonfree-steam rpmfusion-nonfree-nvidia-driver updates-archive
Updating metadata for ‘rpmfusion-nonfree-steam’… done
error: Updating rpm-md repo ‘rpmfusion-nonfree-steam’:
Failed to download gpg key for repo ‘rpmfusion-nonfree-steam’:
Curl error (37): Could not read a file:// file for file:///usr/share/distribution-gpg-keys/rpmfusion/RPM-GPG-KEY-rpmfusion-nonfree-fedora-44
[Couldn’t open file /usr/share/distribution-gpg-keys/rpmfusion/RPM-GPG-KEY-rpmfusion-nonfree-fedora-44]`
Good effort, but once the formatting is removed, it’s removed. Has to be re-added back in by hand. 
Rebase on an Atomic distro with layered repo’s and packages is not possible. A major flaw in the atomic distro’s unfortunately. I tried it last week and got about the same output as you have.
Solution:
Uninstall layered packages (with reboot) and remove the layered repo’s (with reboot)
Rebase
Add the removed repo’s (with reboot) and install the packages (with reboot)
Yes, you will have to do many reboots.
Maybe a clean install is easier to do.
When talking about RPM-Fusion’s free and non-free repos, which are initially installed as local packages, one has to turn them into layered packages, and then rebases to newer releases should be working:
sudo rpm-ostree update \
--uninstall rpmfusion-free-release \
--uninstall rpmfusion-nonfree-release \
--install rpmfusion-free-release \
--install rpmfusion-nonfree-release
However, OPs issue is related to the RPM-Fusion’s repos that are preinstalled (non-free-steam).
Looking at the contents of /usr/share/distribution-gpg-keys/rpmfusion/, RPM-GPG-KEY-rpmfusion-nonfree-fedora-44 is indeed missing. This could be an issue needed reporting.
@frankjunior, RPM-GPG-KEY-rpmfusion-nonfree-fedora-rawhide is there, so you could be able to rebase to Rawhide, but I guess this is not what you want, given that F44 has already been branched.
What you could try is to temporarily disable the rpm-fusion-nonfree-steam repo by editing the file /etc/yum.repos.d/rpmfusion-nonfree-steam.repo and setting the repo to enabled=0. This should only be working if there is nothing layered from that repo though.
No, but the setting is still available under gnome-tweaks.
I hope you understand that f44 is not yet even at the Beta stage.
Problems are extremely likely before it is released and problems should be reported to the developers and not here on this forum. We cannot assist with unreleased versions.