I use Plasma 6.7 so using a Plasma 5 package from the AUR must be a problem, considering the recent attacks, right?
plasma5support is neither a Plasma 5 package nor an AUR package and no, you should not remove it.
What is the meaning of that “5”?
I believe (@arojas may wish to correct/clarify) that this package is/was a transitional support package, to allow certain KDE applications to be able to keep using some of the deprecated KDE5 APIs while they transitioned to becoming fully KDE6-compatible. (Qt has a similar library.) I am not sure what apps still require it, if any; On Gentoo I found it was no longer required by anything I use, I think at least as far back as kde-6.6.0.
I haven’t followed the Arch debacle closely, but I believe it concerned “orphaned” packages that had been languishing without any maintainer for some time, allowing the baddies the opportunity to “claim” and corrupt them. It seems unlikely that would apply to this package. I don’t have a link for you, but I believe a script was made available to check your system against the list affected in this attack.
That said, given what the episode has revealed about the AUR in general, I advise you to examine every PKGBUILD you install, including updates, every time!
Could have a more precise name, like PlasmaLegacySupport or something like that.
It’s supposed to be so some KF5/QT5 apps can work for Plasma 6+. Hopefully this helps anyone not knowing what it’s supposed to be.
No, that’s not what it is. headbank already explained it above.
Hmm… this might only take effect with the Dev build of Plasma 6.8.
dnf search PlasmaLegacySupport
Updating and loading repositories:
Repositories loaded.
No matches found.
Well there’s no such package named “PlasmaLegacySupport”. So I it makes sense that dnf search can’t find it. 
In general this kind of thing is something you shouldn’t bother yourself too much about. Unless you’re a technical expert, the exact packages that make up your distro aren’t super relevant, and removing things without understanding what they are or why you’re doing it is a great way to break your system for no real benefit.
On Arch (and probably Cachy OS(?)) a pacman -Qdt would list packages that were installed as dependencies but are no longer required by any other installed package. (* with the asterisks to “see the last post, from ngrahan, second paragraph”).
plasma5support is not one of them, and not a AUR package either as arojas already said.
According to a pacman -Qi plasma5support, that lists information about a package on Arch, it is still required by kscreen, plasma-desktop, plasma-welcome and wacomtablet on my up to date Arch (plasma 6.7.4) system at least.
Not trying to break this, but it just, well I am seeing if the change happened. I’m on Kernel: 7.1.4-204.fc44.x86_64 arch: x86_64 bits: 64 compiler: gcc v: 16.1.1
Desktop: Cinnamon v: 6.6.7 tk: GTK v: 3.24.52 wm: Muffin dm: LightDM
Distro: Fedora Linux 44 (KDE Plasma Desktop Edition)
Machine:
as INXI says. I only am using a diffrent desktop temporaily as ummm… I got bored. yeah. I don’t know why.
Eh… Same here actually, now I look. Checking logs and looks like it existed along with every Plasma version I’ve installed. Sorry for the hallucination… 