I'm also having this issue...
I suppose we should not try to fix it on out local machines. It is being addressed elsewhere. Right?
root@DC7100-HP:~# uname -a
Linux DC7100-HP 4.19.0-12-686 #1 SMP Debian 4.19.152-1 (2020-10-18) i686 GNU/Linux
root@DC7100-HP:~# apt update
Hit:1 http://deb.debian.org/debian buster InRelease
Hit:2 http://deb.debian.org/debian-security buster/updates InRelease
Hit:3 http://deb.debian.org/debian buster-updates InRelease
Err:4 https://repo.sparkylinux.org core InRelease
403 Forbidden [IP: 193.107.89.176 443]
Err:5 https://repo.sparkylinux.org nibiru InRelease
403 Forbidden [IP: 193.107.89.176 443]
Hit:6 http://www.deb-multimedia.org buster InRelease
Reading package lists... Done
E: Failed to fetch https://repo.sparkylinux.org/dists/core/InRelease 403 Forbidden [IP: 193.107.89.176 443]
E: The repository 'https://repo.sparkylinux.org core InRelease' is no longer signed.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.
E: Failed to fetch https://repo.sparkylinux.org/dists/nibiru/InRelease 403 Forbidden [IP: 193.107.89.176 443]
E: The repository 'https://repo.sparkylinux.org nibiru InRelease' is no longer signed.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.