I have been in the R/C industry flying planes and helicopters for more than 25 years. 99% of them have been all-electric. I was among the first that was using lithium battery cells at 5C discharge rates flying my tiger moth. It went from flying for 6-minute flights using nicad to having 55-minute flights using lithium. No charging protection, no discharging protection, these were bare cells just taped together. You learned by trial and error back then and many folks ended up with burning planes due to over-discharge.
Batteries today are far better than the past. Lots of protections on them. It's one thing to have it catch fire in an r/c plane, it's another to have your cell phone catching on fire in your pocket. With that said, Yes Lithium hates cold. In the R/C world, It cold take my Sukhoi from 8-10 minute flights all the way down to 2-minute flights. They really start hating weather when it gets below 55 degrees F.
Heat is truely only a major factor for charging. It can be 100 degrees and in most cases, it is not going to affect them in the slightest, UNLESS ithe device is under severe load. The problem arises when you try to charge a hot battery pack without letting it cool down. That is where you will get your battery degradation. The only way to harm a battery by leaving it plugged in is if the charger protection fails. Its true lithium does like to get worked, However, Lithium is not like other batteries. It setting at 100% is not going to hurt that battery. The charging would do far worse than it just sitting there. I don't have that tiger moth anymore, Retired it probably 20+ years ago. I still have that 5c battery pack to this day. It's useless for any r/c things today but it still amazes me at just how well that battery has held up over the years.
I also have a Nissan leaf 2013 (No thermal management at all on the battery for cooling) that is still at full bars. Technically it's down to 89% battery capacity but thats where it was at when I bought it 3 years ago and it hasn't moved. My only charging requirements is I will only charge it once I get below 60% usage. I will charge it up to full and it can set there for weeks sometimes and never get touched and the battery is perfectly fine. Lithium is one of the most resilient battery tech innovations we have had in my lifetime. It's far hardier than people give it credit for and has some harsher sides when dealing with cold but there is also a lot of bad information out there that people for some reason still believe and they usually are only true for older generations of battery types.