Ever since the GOG website browser changed a while ago, I don't think GOG does series discounts outside of promos. Is this true?

There are series discounts, but nothing indicates that it's a series discount unless you check the game prices and do the math.

You can experiment with that by clicking that buy entire series button, and then trying to remove some games from the cart and checking what happens to discounts.

They are still there, just have displaying glitches.
https://www.gog.com/forum/general/gog_buy_series_discount_list