Why did GOG stopped putting anymore extra goodies in their games since 2015 or so?

Extras depends on the publisher, not GOG . + Indies realies that they can earn more money by selling the OST separately( dont mind it.. as long as we see the OST on GOG and not only on Steam)

Its so they can sell them separately (not necessarily GOGs fault, although they allow the practice). It started with soundtracks, now its manuals, comics, pictures, anything in fact. Its why you see several different packages for the same product. This is "advancement" and "moving with the times" so I have been reliably informed.

We have gone over this before, not a single game has it's manual sold separately.

And a link posted in that thread showed there was indeed one product where manual was a separate item.

Edit:
Ah, here we go, the original discussion:
https://www.gog.com/forum/general/vikings_wolves_of_midgard_game_guidemanual

And the direct link to the product in question:
https://www.gog.com/game/yookalaylee_digital_deluxe_edition_upgrade

So, to clarify Digital Manual + Soundtrack is being sold for this product separately for the princely sum of £7.99. I can't tell you if there are more, but there is one at least.

It doesn't say digital manual. It says manual at book

Quote directly from that page:
Yooka-Laylee’s 24-page digital manual and art book features exclusive details on the characters, worlds and challeng

If u 1 me put it simply, i wld say it all boils down 2 profit aka money. If they can sell the extras as an additional 2 the game instead of bundling them together with the games y not? I'm sure they wun mind making more money of cos just like every1 wld.

While i do hope that GOG can continue 2 add the little extras that come with some games which is 1 of the reasons y i prefer buy my games here over other platform (other than the games here being DRM-free of cos), i also uds the game developers/publishers may not allow that as can be seen fr the many diff version of the same games (digital, premium, special, limited & so on). Personally, if i feel the extras r not worth that extra money i wld only go 4 the base game but i may make exception 4 soundtrack as i'm a sucker 4 gd music. Lol.

I hope this helps ans ur qn. :)