The only game I could find that was arguably a third party exclusive for steam was Darwinia in 2005, and they weren't paid by Valve to do so, which pretty much scuttles the comparison.
If you meant first party exclusivity then sure Valve does that with their own games, as does Epic and others. First party exclusivity isn't the problem here though, it's the third party kind instead.
The only game I could find that was arguably a third party exclusive for steam was Darwinia in 2005, and they weren't paid by Valve to do so, which pretty much scuttles the comparison.
If you meant first party exclusivity then sure Valve does that with their own games, as does Epic and others. First party exclusivity isn't the problem here though, it's the third party kind instead.