SUMMARY

Package managers are unable to fetch Release file in http://archive.neon.kde.org/user bionic repository. 

STEPS TO REPRODUCE

1. apt update or pkcon refresh
2. Notice error
3. Only happens when apt/pkcon actually checks http://archive.neon.kde.org/user bionic Release

OBSERVED RESULT

When using apt:
...
Err:26 http://archive.neon.kde.org/user bionic Release                                                                                                                                                                
  504  Gateway Time-out [IP: 195.181.169.6 80]
Reading package lists... Done
E: The repository 'http://archive.neon.kde.org/user bionic Release' no longer has a Release file.
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.

When using pkcon:
Refreshing cache              [=========================]         
Loading cache                 [=========================]         
Downloading packages          [=========================]         
Running                       [=========================]         
Finished                      [=========================]         
Fatal error: E: The repository 'http://archive.neon.kde.org/user bionic Release' no longer has a Release file.
W: Updating from such a repository can't be done securely, and is therefore disabled by default.
W: See apt-secure(8) manpage for repository creation and user configuration details.
E: http://archive.neon.kde.org/user bionic Release is not (yet) available (504  Gateway Time-out [IP: 195.181.169.6 80])

EXPECTED RESULT

Notification that all packages are up-to-date or a listing of which packages need to be upgraded. 

SOFTWARE/OS VERSIONS

Operating System: KDE neon 5.14
KDE Plasma Version: 5.14.5
Qt Version: 5.11.2
KDE Frameworks Version: 5.54.0
Kernel Version: 4.15.0-45-generic
OS Type: 64-bit
Processors: 8 × Intel® Core™ i5-8259U CPU @ 2.30GHz
Memory: 7.7 GiB of RAM

ADDITIONAL INFORMATION
Thread on forums:
https://forum.kde.org/viewtopic.php?f=309&t=156839