Insert the following lines right above [extra] in /etc/pacman.conf:
[plasma-5.24-lts]
SigLevel = Required DatabaseOptional
Server = https://manjaro.github.io/plasma-5.24-lts/x86_64/
Install Plasma LTS on your system
sudo sh -c "pacman -Syyuu"
Voila! You should now see Plasma LTS on your system.