It likely requires the dependency LibClassicMobHealth-1.0, you can find it on CurseForge here: https://www.curseforge.com/wow/addons/libclassicmobhealth-1-0, I have confirmed this working for SimpleUnitFrames Classic.
Thanks for being here, @NoobTaco. If you guys are looking for addons, be sure to head over to willitclassic.com for your addons needs! you can easily find the addons you want and search using a clean interface!
You can glean a lot of information from these changes (and the link to the actual code Diff) about what's changed between the 81000 interface (not 73500).