Dug around a bit and here's the sit about the append:
If I got things right, the dev released the append on 2022.01.25, which made its way to DLsite and FANZA that day, and had plans to have a way for people to buy the 1) "Complete" ("Integrated") version (base game + append), the 2) the append as standalone but which could also be applied upon 3) the standard (back then v1.13) game. Prices would have been 1)=2)+3).
So there should have been 3 possibilities to choose from at the beginning of Feb., but shit and DLsite policies happened and only the "Complete" version is available now (and I guess, for the foreseeable future) from a couple of weeks back.
They gave Kagura the standard (base game version) to work with (no idea when that was, though), and said about the KG/Steam release on their ci-en
"Steam版について
Kagura Games様にまずは通常版をお渡しし、翻案等々をお任せしている状況です.
通常版のリリース時期やアペンド版の取り扱い如何については未確定です。詳しくはKagura Games様にお問い合わせください."
"About the Steam version
We have given the standard version of the game to Kagura Games, and are leaving the adaptation to them.
The release date of the standard version and how to handle the Append version are yet to be determined. Please contact Kagura Games for details."
Also, as of today (
02.26)
"DLCを含む完全版の翻訳と取り扱いについては
KaguraGamesさんと相談中なので、続報お待ちください。"
"As for the translation and handling of the full version including DLC
We are currently discussing the translation and handling of the full version including DLC with KaguraGames, so please wait for further news."
So tough luck, I guess. Though not entirely hopeless, I'd say.