I think the 80% thing has came from other manufacturers recommendations and been inadvertently shared by others through YouTube etc so has kind of became a standard. I’ve also found “regenerative breaking limited” warnings when above 80-85% charge but I can’t 100% say this is down purely to battery level so that may also have been brought in when people say 80%
As you say the Tesla manual recommends charging to within the daily range on the battery screen (I think this is between 50-90% but can’t remember off hand) I leave mine set at 90% as I’m so far only charging while out and about as I do very little actual driving since getting the car in September - a top up once a week down local park while walking is enough for me and saves me paying for electricity
When I start using it more daily I’ll have it set to enough to allow me to do what I have to do with some spare - purely to avoid paying for electricity unnecessarily
You’re unlikely to damage the battery beyond normal degradation unless you routinely charge to 100% and leave it there for days. There’s plenty of social media people over in USA who aren’t car fanatics who only charge at super chargers several times a week and so far none of them are complaining their batteries are fried
Also if you have the free supercharger miles - don’t avoid using them as I’ve ended up only using 200 odd and lost the other 800
