I can attest that the blade battery doesn't seem to care if you take it all the way to 100% or drop it as low as 5% regularly. I've had my car for over 3 years now, and the battery degradation has been negligible. I've lost 1% over all this time, and both our cars (BYD HAN and Tang) are consistently allowed to drop under 10% before we decide to go charge them back to 100%. Granted, we live in the Caribbean, so we don't have to deal with cold weather ever.

Edit: This was all wrong. I forgot I have a battery saver mode on my phone that lowers "fully charged" to something like 80%, so it is ideal to keep it "fully charged".

Never heard the "above 80%" thing. I'm pretty sure you're wrong about this. With lead-acid batteries, this was optimal. I'm pretty confident that lithium ion batteries it's best to keep the charge as high as possible. Ideally you'd only ever use it fully charged. It's health is harmed by draining it low/fully.

I don't own an EV, but I know enough about it that I'm pretty sure this is the case. You should look it up for your vehicle though. This advice also applies to phones and other lithium ion batteries too. Lead-acid was damaged by keeping the charge high, but lithium ion is damaged when low, and almost all devices are lithium ion now.