A sudden drop to 10.8ish v is almost always a sign of a dead cell. The other 5 cells may still be working perfectly fine, and 10.8v is enough for non-sensitive loads, like lights. I have a few old batteries with bad cells I'm keeping for outdoor lights (with a buck converter to 3.5v LEDs). I wouldn't use an expensive charge controller on them as they may damage it(?), but they can still be useful for some things. It sounds like you got a few good years out of it despite not having enough solar to charge it properly. If you had hooked it to an AC charger, just once a month, it probably would have greatly prolonged it's life.
What the heck is a LB?
LB is leisure battery.
I got 2 years and 4 months of full-time use out of it.
I think this is how I messed up.. During summer while parked partially in the shade, using my water cooker a few times in the evening, then in the morning while not paying attention to the BM1 and MT50 displays anymore. I got lazy. There was full sun all day long anyway and the display never showed values lower than 12.8v, 12.7v at the end of the night so what gives?
Yet, suddenly (to me) the Victron shut off all power and the display read 10.6v. This happened a few times. I suspect that if I hadn't used the watercooker without paying attention to the battery SoC, it would still be okay (aside from the systematic undercharging). But that's in hindsight.