How to downgrade linux-lts back to 5.10 on Arch Linux/Manjaro from 5.15, and keep it there!

First, you will need to edit /etc/pacman.conf to prevent linux-lts from updating every time you do a system update.

sudo vi /etc/pacman.conf
# sudo nano /etc/pacman.conf

IgnorePkg   = linux-lts
IgnorePkg   = linux-lts-headers

Add two IgnorePkg lines with linux-lts and linux-lts-headers. That will “hold” the package back.

Then, downgrade linux-lts to the last and final release:

sudo pacman -U https://archive.archlinux.org/packages/l/linux-lts/linux-lts-5.10.90-1-x86_64.pkg.tar.zst
sudo pacman -U https://archive.archlinux.org/packages/l/linux-lts-headers/linux-lts-5.10.90-1-x86_64.pkg.tar.zst

Then reboot!

5.10.90-1 is the final 5.10-lts, as you can see here: https://archive.archlinux.org/packages/l/linux-lts/