No real reason to keep them, if you've already been able to boot into a newer one and haven't had problems with it that might necessitate going back to an older one. But at the point you can remove them, you can also reinstall them if that comes up.

They shouldn't cause any problems or vulnerabilities if they aren't what your system is running, but I removed them anyway

If you aren't booting that kernel it doesn't really matter either way as far as I know. If you aren't using them they will just take up space anyway, I could see having one or two for backup but fwiw the default on Arch is to only have the current kernel unless you install extras. If you haven't noticed any major glitches in a week or so I would think it's pretty safe to prune the old kernels.

You'll eventually get notifications telling you you're running out of disk space on that partition and the solution is to delete all your old and not utilized kernels. Just delete them now and every once in a while that you remember to