Well 20k since 2018 means they sure did ride that bike. There was a video of a guy with an Elcon in a top case on a FXS might be the same guy.
As to warranty in the US a company has to prove a aftermarket item caused damage to void the warranty.
No one can say if your bike will have problems with it at the 20k mark it's really a roll of the dice. Your charger may die tomorrow or last 6 more years. Not trying to scare you off but it's just the truth. The bearings and springs will have more wear on them so keep that in mind.
In my years of owning (33k on mine) and reading other people's comments on their high mileage bikes it's mostly fine.
I would say I'd rather have a 2016 bike with 15k miles on it than a 2015 with 2k. So far it seems the battery "likes" to be used and recharged rather than just sit.
I would advise having the money to cover a few costly repairs just in case. It's not a deal if it has issues that can't be fixed and it ends up sitting in a garage.
Good luck. People say they are really fun bikes.