Is there any way to subscribe to receive emails about Steam games offered at 50% or higher discount?
i think this site could help.
Apparently you can register and if some games go on sale you get notified by e-mail. ;)
You can register to Steam Forums and sign to receive notifications for the following thread answers: Steam Deals and Price Reductions!
I follow @steamydeals on Twitter. It's inofficial, but it does a good job of capturing all the offers and pricedrops, one per tweet, including the new price and the reduction, and including a link to the store page.
That's all offers and pricedrops, though, not just those with 50%+.
There are (3rdparty) tools for getting new tweets by mail, but I don't know much about them.