Yeah, in fact :point_down.
ID Branch Op Remote Download
1. [✗] org.freedesktop.Platform.GL.default 21.08 i flathub < 129,8 MB
2. [✗] org.freedesktop.Platform.Locale 21.08 i flathub < 326,8 MB (partial)
3. [✗] org.freedesktop.Platform.openh264 2.0 i flathub < 1,5 MB
4. [✗] org.freedesktop.Platform 21.08 i flathub < 202,1 MB
5. [ ] com.alientrap.nexuiz-classic stable i flathub < 884,5 MB
Warning: Can't pull from untrusted non-gpg verified remote
Warning: Can't pull from untrusted non-gpg verified remote
Warning: Can't pull from untrusted non-gpg verified remote
Error: Can't pull from untrusted non-gpg verified remote
error: Failed to install org.freedesktop.Platform: Can't pull from untrusted non-gpg verified remote
Also, the procedure you suggested didn't work.
luca@Asus:~$ sudo cp /var/lib/flatpak/repo/flathub.trustedkeys.gpg /var/lib/flatpak/repo/flathub.trustedkeys.gpg.bak
[sudo] password for luca:
luca@Asus:~$ wget https://flathub.org/repo/flathub.gpg
--2024-07-14 17:56:06-- https://flathub.org/repo/flathub.gpg
Resolution of flathub.org (flathub.org)... 2a04:4e42:4b::347, 199.232.57.91
Connection to flathub.org (flathub.org)|2a04:4e42:4b::347|:443... failed: Connection expired.
Connection to flathub.org (flathub.org)|199.232.57.91|:443... connected.
HTTP request sent, awaiting response... 301 Moved Permanently
Location: https://dl.flathub.org/repo/flathub.gpg [more]
--2024-07-14 17:58:21-- https://dl.flathub.org/repo/flathub.gpg
Resolution of dl.flathub.org (dl.flathub.org)... 2a04:4e42:7d::347, 146.75.53.91
Connection to dl.flathub.org (dl.flathub.org)|2a04:4e42:7d::347|:443... failed: Connection expired.
Connection to dl.flathub.org (dl.flathub.org)|146.75.53.91|:443... connected.
HTTP request sent, awaiting response... 200 OK
Length: 2844 (2.8K) [application/octet-stream]
Saved in: 'flathub.gpg'
flathub.gpg 100%[===================>] 2,78K --.-KB/s in 0,003s
2024-07-14 18:00:36 (797 KB/s) - ‘flathub.gpg’ saved [2844/2844]
luca@Asus:~$ sudo cp flathub.gpg /var/lib/flatpak/repo/flathub.trustedkeys.gpg
luca@Asus:~$ flatpak install nexuiz-classic
Looking for matches…
Found ref ‘app/com.alientrap.nexuiz-classic/x86_64/stable’ in remote ‘flathub’ (system).
Use this ref? [Y/n]: Y
Required runtime for com.alientrap.nexuiz-classic/x86_64/stable (runtime/org.freedesktop.Platform/x86_64/21.08) found in remote flathub
Do you want to install it? [Y/n]: Y
Info: runtime org.freedesktop.Platform branch 21.08 is end-of-life, with reason:
org.freedesktop.Platform 21.08 is no longer receiving fixes and security updates. Please update to a supported runtime version.
Info: applications using this runtime:
com.alientrap.nexuiz-classic
Info: runtime org.freedesktop.Platform.GL.default branch 21.08 is end-of-life, with reason:
org.freedesktop.Platform 21.08 is no longer receiving fixes and security updates. Please update to a supported runtime version.
Info: applications using this runtime:
com.alientrap.nexuiz-classic
com.alientrap.nexuiz-classic permissions:
network pulseaudio x11 devices
ID Branch Op Remote Download
1. [✗] org.freedesktop.Platform.GL.default 21.08 i flathub < 129,8 MB
2. [✗] org.freedesktop.Platform.Locale 21.08 i flathub < 326,8 MB (partial)
3. [✗] org.freedesktop.Platform.openh264 2.0 i flathub < 1,5 MB
4. [✗] org.freedesktop.Platform 21.08 i flathub < 202,1 MB
5. [ ] com.alientrap.nexuiz-classic stable i flathub < 884,5 MB
Warning: Can't pull from untrusted non-gpg verified remote
Warning: Can't pull from untrusted non-gpg verified remote
Warning: Can't pull from untrusted non-gpg verified remote
Error: Can't pull from untrusted non-gpg verified remote
error: Failed to install org.freedesktop.Platform: Can't pull from untrusted non-gpg verified remote
Now, since it failed, why should I use flatpak remote-delete flathub? It looks like that it will remove all flats that I installed, and I can't, they're mostly GBs and GBs of my games 
.