Another game called Hades has Steam cross save option and the page still works on that (just checked myself). CDPR should be able to update the link of Steam authentication I'm sure, otherwise if it was whole broken by Steam, it wouldn't work on Hades either.
Sure ir work for other games...
But CDPR now use GOG for Cyberpunk and The Witcher 3 (which is their own store) for cross-progression... So I guess (just a guess^^), Steam decided shortly before the release of the next-gen update* (which include the GOG cross-progression on all platform except on Switch) to stop to provide this feature for The Witcher 3.
And I said "Steam broke it" because the link in the game didn't change since the last update on Switch, years ago... If it doesn't work, it mean that the page it was pointing to (on Steam servers), does not longer exists (hence the message > this page cannot be displayed)
*Strangely, Steam cross-progression on Switch stopped to work about a month before the next-gen release, around the release date announcement... pretty weird coincidence, isn't it ?
Again, an update is planned on Switch, which will use GOG cross-progression like other platforms... So useless for Steam or for CDPR to even fix now.