MarsSeed [1] filed a request to merge plasma5-applets-thermal-monitor-
git [2] into plasma6-applets-thermal-monitor-git [3]:

Only the plasma6 package works now on KDE Plasma 6.

[1] https://aur.archlinux.org/account/MarsSeed/
[2] https://aur.archlinux.org/pkgbase/plasma5-applets-thermal-monitor-git/
[3] https://aur.archlinux.org/pkgbase/plasma6-applets-thermal-monitor-git/