If you still want to track down a 2015 build for legitimate preservation or offline study, I’d recommend checking:

Before the drama, before the critics, before the constant delays of the "Osana" rival, 2015 was when Yandere Simulator was just a fun, janky experiment. Many YouTubers (like Markiplier, Bijuu Mike, and Kubz Scouts) first played the game in 2015, creating a sentimental attachment.

The developer does not officially support or host old builds. You will not find these on the official YandereSimulator website (which only hosts the latest build).

Communities like r/Osana or various Fandom wikis often have users who maintain Google Drive backups of the oldest builds.