There's a technical part and a more "realistic" part. The tech technical part is than when Lithium-ion cells are over 82F, then degradation begins and it accelerates the hotter the battery gets. All the way past 140F where you start to get cell failures. This also happens when the cell is charged past 80% of it's lab rated capacity and the closer to 100%, the more degradation you get.
Now the more realistic part is that the degradation is rated over +20 years (maybe more, can't remember exactly what the all the research papers said about that specifically), so that means that in a perfect environment where the temperature is always perfect, the charge and discharge cycles are always perfect, spaced out, no extra wear and tear on the cells, they can run for +20 years non-stop. So what happens if you leave your Leaf at 100% SoC (state of charge) for days, weeks, months? You eat into that battery life, slowly, over time. So instead of the battery lasting 20 years, it might last 19, 18, 17, etc. years depending on how long it sits at 100% for long periods of time. The same thing with the heat. If the battery is always over 82F, then that eats into the battery life as well.
So the balancing act is, if you drive you Leaf daily, then don't worry about it being at 100% SoC, but at the same time, don't take a 2 week vacation and leave it sitting at home with 100% SoC during that time. The battery won't die instantly, but if you plan on keeping the Leaf for a long time, you will help the battery life. The same thing with the temperature. If you QC (quick charge) a lot, try to plan it during a time when the battery is the coolest. I do almost daily QC on my Leaf, but I try to time it for the morning when the battery is cool and even after a QC session, the temperature might never get over 82F. Even if it does, it's more time for it to cool down than if I did the same thing in the middle of the day when it's hotter.
Finally, sometimes real life means you can not always time your charging perfectly. So if you leave the Leaf at 100% SoC an extra day due to a real life emergency, don't sweat it. If it's not possible to time all you QC sessions for perfect weather every single day, don't fret the small things. Individually, they are noting to worry about, but added together, they can make real issues. If you always leave the Leaf at 100% SoC over the weekend because you don't want to worry about plugging it in the night before work, that adds up. You are using QC every day to take advantage of free charging when you always have plenty of range to make it home, that also adds up over time that could become a bigger issue.
Hopefully this can help temper any worries you have about battery care. Do be aware that you should take care of your battery, but don't obsess over it.