A few years ago there seemed to be good deal on same series games from the same publisher. I'd imagine GOG sold more by having these deals. Now they've disappeared, which is a shame I think. Any chance they'll come back?

They still exist, but they are invisible.
Like many other "features" with the new design, hiding those discounts doesn't seem to make much sense.

You can experiment this by adding some game series into your shopping cart, going to the checkout page, and then deleting something and seeing what happens to discounts.

Note that this doesn't work with all series, and during sales temporary discounts probably override series discounts.


This list may, or may not, show currently existing discounts:

http://www.gog.com/forum/general/gog_buy_series_discount_list/page1