Something like this was part of the game in the past, but was removed after two month of OP (I'm not sure if you played already back then). While the removal of gold immunity was quite controversial at that time after a while a large majority of the community seemed to approve of that change.
The main reason why gold immunity was removed was because it made the game a lot more draw dependent and uninteractive. If one player played a gold card and the other one didn't have any counter for it, there was nothing he could do. Yen:Con was one of the strongest examples (basically imagine her +immunity now). Or times, when a player just kept all his golds until round 3 and the opponent couldn't do anything about it. In Closed Beta there was even a NR archetype which had the goal to make most of your cards gold during the game and thus pretty much safed your points from removal.
Of course there were a lot of things different, silvers weren't golds back then, so the actual number of cards that you could have to be gold or to interact with golds was much smaller. And with the variable amount of gold cards now, it would spice things up even further. And moreover Shackles removed the gold status from cards back then, which means that Shackles were the best not gold counter against golds.
While things have changed, I believe readding gold immunity is still a bad choice, because it would add another different group you need counters against and would make the game even more removal heavy in my opinion. Have no gold counter in hand and the enemy plays some high rolling gold engine then you have lost already. We know that artifacts are uninteractive already, weather as well and both have been quite disliked therefore. Bringing golds in the same situation doesn't make that better.
But on the other hand, such a mechanic is already partially included in the game. If we look at bronze versus gold engines, most gold engines have no range or a far larger range than bronze engines and are therefore harder/riskier to remove. The problem is that the row system isn't really used to its fullest and therefore the difference isn't that visible.
To improve a soft immunity for golds in this way could be more cards that only target bronze cards. Though to balance that they would probably overkill any bronze engine instantly, which isn't good either.
One of the problems that you overlooked in the suggestion in my opinion is also, that most gold cards are a lot more highrolling and therefore reach a lot less value compared to their provisions if countered. Seltkirk being countered by a Drowner is completly designed this way, else there wouldn't be a reason why he was restricted to melee. And moving is something that can be easily recountered by just moving him back with the right cards (which aren't as good/as available to some archetypes which is another problem)
Last but not least I wouldn't have a problem with some restriction for bronze cards to damage golds or on the other hand more ways to protect gold cards, but I doubt that gold immunity is the right choice.
