Was it removed? Any way to download it now?

Same issue here, it seems they removed the 15th Anniversary edition from the Extras page. I'll try getting in contact with support to see if I can snag a download.

Any luck?

Looks like it's not just 15th Anniversary edition was removed but other extras as well.
* "Making Of" video clips with Spanish and Polish subtitles are missing.
* Design.pdf in 7 languages with early art drafts by Eric Chai is missing.
* Technical Book.pdf with some cryptic records of the technical innards of the engine by Chai is missing.
* Wallpapers are missing. But that's fine since they are really nothing special and a far from being beautiful.
* Soundtrack in MP3 and FLAC is no longer available. But this one is fine too since we still have WAV version.

(TL;DR: No.)
Finally got a reply back (after forgetting to email in the first place), and they said the 15th Anniversary edition was "replaced" by the 20th Anniversary edition, and I should be able to download all the goodies, despite the 15th Anniversary edition never being sold in the first place on GOG, pretty sure, and it seems like PCGW merged the articles for the game's different releases. I'll still try & see if I can get a link, but it seems unlikely.