A lot of people say different things about the lithium ion batteries. It makes for a lot of confusion.
Whatever they say different there are two things guaranteed to kill a lithium battery:
1: Deep discharge to silly low voltage. Most phones have protection circuits to shut down before the battery is irreversibly damaged.
2. Overcharging. This can cause thermal runaway and a spectacular fire or exploding phone.
It seems to be a widely held opinion and it certainly aligns with my experience that deep discharge (to shutdown level) usually has a disproportionate hit on battery life. You won't see it fron one or two flat battery events, but if it is a regular occurrence the battery capacity will degrade faster than by keeping it topped up when possible. I frequently charge in the car so my batteries are usually well charged, and I have several batteries 3-4 years old still going well.