It's not the case. Today Quicksilver offered to update the plugins I use and all of them failed to install with the same error as OP.
I tried to download them from the website (
https://qsapp.com/plugins.php) and installing them manually - after doubleclicking a .qspkg and selection "Install", Quicksilver indicates "The file is most likely corrupt" for all the plugins I tried (list below). Please not I do not use the email support plugin.
Calculator Plugin 2.3.0.qspkg
File Compression Plugin 1.8.0.qspkg
Path Finder Plugin 1.2.0.qspkg
Contacts Plugin 2.3.0.qspkg
Keychain Plugin 1.2.0.qspkg
Safari Plugin 2.5.0.qspkg
FaceTime Plugin 1.2.0.qspkg
Networking Plugin 1.4.1.qspkg
Regards,
WJ