That is not possible as KDE Plasma 5 is no longer in the repo.
Do as @cscs suggests - adapt to the new Plasma - it is easier than you think - and the result is far better than you expect.
portable mirror
I wrote a topic on May 10. about creating a portable mirror and I still have the copy 
of stable branch dated at May 10.
unsupported
I am just being creative here - and I wouldn’t do it if I were in your place - but hey! it is your system - just don’t come back and complain.
But if you insist you can try to find a mirror which has not updated since before May 13
Remember Dont complain
Use https://repo.manjaro.org and filter the list to only include out of sync mirrors e.g. this one - which has not synced since 5139:55 hours and counting. Checked at 2024-05-21T06:37:00Z - the mirror has not synced for 214 days and if you average a month to 30 days - this ~7 months - so don’t be creative.
https://mirrors.piconets.webwerks.in/manjaro-mirror/
If you still insist on being creative - set the mirror as your primary mirror
sudo pacman-mirrors -aU https://mirrors.piconets.webwerks.in/manjaro-mirror/
The run
sudo pacman -Syuu
tuxboy.eu archive
There is an unofficial archive of Manjaro packages - but this also requires a tremendous amount of work to be able roll back your system - it is only served over http