This package's original identifier was com.atteneder.ktx (see Unity Fork for details). You can still use/install it with that identifier.
Via OpenUPM
The recommended way to install is to download and open the Installer Package
It runs a script that installs KTX/Basis Texture Unity Package via the OpenUPM scoped registry.
Afterwards and further, optional packages are listed in the Package Manager (under My Registries) and can be installed and updated from there.