Hi. Welcome to the community. 
If you would like to keep plasma 5 temporarily, then the main thing you need to do is not update your system for a few weeks.
If it is that you would like to keep plasma 5 for more than a month, or even more, like a year, then you shouldn’t be using an Arch-based Linux distro.
Now, not updating your system has its flaws, mainly security flaws if you use your system to connect to the internet. In addition to this, it requires more care to ensure you don’t update the system while still updating the apps. You can achieve this using any of the methods below, but it is only recommended for users who understand how pacman works, and how to maintain their system in general.
Excerpt from: KDE Users Should've Waited
For info on updating to KDE plasma 6 and avoiding issues, please see: Plasma 6 Update some hints
NOTE: If you are not a techie, you should really avoid not updating your system, as it is more likely to cause a ton of issues down the line.