Just the cached packages - as in you would not be able to downgrade locally or manually install from there and instead would need to download a new copy. It would have no affect on your actual system beyond that functionality.
And it will refill next time you update, or for each new package you install.
Oops. I should have also mentioned the tool for the job when we take care of cache
(I had assumed if pacman cleaning wasnt working we would need to use our hands…)
paccache -rvk3
This would remove everything but the last 3 versions in a cache.
Similarly you could remove everything there, almost identical to what we did above, with