This is the official Repository for BetterChairs provided on SpigotMC (https://www.spigotmc.org/resources/better-chairs.18705/).
Good first step! But there is no real official download here. The one writing this is @Sprax2013 and I have gained access to the project by the original Author @BlackScarx.
BlackScarx has been inactive for quite a while and I try to take over this project officially.
A newer version supporting 1.15.2 and other versions can be found at https://github.com/Sprax2013/BetterChairs/releases.
BUT PLEASE keep an eye on the SpigotMC-Page - I'm still trying to contact the original author.
Hard to say, as the original author is sadly inactive...
I wrote the currenty status down at BlackScarx/BetterChairs#15.