Hi Kristin, I'm no expert but I just joined the forums and see your question hanging unanswered, so I did some searching (I have an unused library science degree that I hope to do justice to, lol)
It has successfully downloaded to my Mac but I already have a newer version of Soulseek running so don't want to test it out. I am more of a Linux person than Mac, but my hopeful *guess* would be, if you install this older build on your computer, it would then allow you in-app to upgrade from that version to whichever the latest version your OS *can* handle would be. Fingers crossed!
From my cursory searching, Soulseek doesn't provide a user-facing repository of versioned older builds where you could navigate to the exact perfect one for your situation, but if the 8-years-old installer still, in fact, installs, I hope that could help you along. Final note- the second URL linked in that forums discussion above seems to now direct to some sort of spam site, so clickers beware!