The warranty is like a year if I recall, it's not much. Used to be more but they actually dropped it a few years ago. This particular one is from 2013. I would expect to lose some charge but not just outright die this early.
That battery has A 1 YEAR WARRANTY.
Every day after 1 year, is a bonus.
If this battery was born in 2013, then I would expect it be dead in 2019.
Yes, the plates have shed and shorted one cell.
Yes, one day it goes from 12.8 Volts, then down to 10.6 volts = shorted cell .
How often do you "Deep Discharge" this battery?
How many Deep Cycles has this battery provided in 6 years?
Is your avg Depth of Discharge = 50% ?
How many Ah's consumed per deep discharge
It is only a 80Ah battery, so 50% DOD = 40Ah which is only 4 hours at 10 amps.
Batteries die from use and non-use ( aging )
Even if you do not use a Lead Acid battery ( Float Charge ) it will not last forever.
3 - 5 years is typical for a Marine-Hybrid type battery
The Plate thickness is not based on volts nor amp-hours
Plate Thickness ...
0.040" = Automotive SLI battery ( sponge lead )
0.070" = Marine-Hybrid Battery
0.110" = GC2 Golf Cart
0.265" = Deep Cycle Rolls-Surrette Battery ( paste lead )
Many thinner sponge lead plates is for starting cars.
Fewer thicker paste lead plates is for deep cycling
There is no hard line between what is a "car" battery and what is "deep cycle" battery.
There is a huge range of plate thickness, ratings and naming conventions, in the middle
Typically, any plate thicker than 0.100" ( GC2 Golf Cart ) is considered a "Deep Cycle" battery.
The thicker the plates, the better for Deep Cycle applications.