Which package should be removed, and why?

syncthing was removed in this commit on 2019-01-20 (about 5 months ago). However, the old/abandoned version in [desktop] is "shadowing" the updated version in the CCR.

For example, pacman -Ss syncthing still shows the outdated version (along with qsyncthingtray and syncthing-relaysrv). Furthermore, both chaser update and the equivalent ccr -Syu --ccronly completely ignore the CCR version. As a workaround, it's necessary to run chaser install syncthing or ccr -S --ccronly syncthing to switch to the CCR version, and again on every update.

Disclaimer: I am the current maintainer of the CCR Syncthing package.

Related: #113 (closed), where an outdated Syncthing was reported and its removed status clarified. It seems that everyone except the repo knows that it's been removed. ;)