I've had my S8+ for 26 months and the battery seems to have degraded a lot. Once it gets to about 30% left, it can power off without warning or quickly drain to 15% or so and then power off.
I purchased it for $1330 from mobileciti so just feels bad to have paid so much for it and now it looks like it's degraded. I'm assuming my options for warranty are little to none? I'm not sure if this ACCC talk of a "reasonable" period of time applies here [where warranty can still be valid despite it technically ended, if consumer paid a certain amount for a product).
Little to no chance of any warranty repair, even within warranty, it would be tough. Your best bet is to just pay to have the battery replaced.