I tried to install xdebug/xdebug as is shown in the docs and xdebug/xdebug:^3.4 as is shown in the CLI example but received the message:

I believe the problem is that there is no stable version but this message is identical to if the package doesn't exist at all.
The message could perhaps read something like: "Unable to find a stable version of xdebug/xdebug, try installing a specific version."