We have good data on 8 year old batteries, but the batteries have changed much since 2013, increasing in size, including better thermal management, and the batteries tend to use less of their total % range for journeys.
Manufacturers already 8 year warranties on the traction batteries that promise at least 70% of the original capacity. Bjorn Nyland did some videos of EVs in Thailand where the EVs are in a much hotter environment & he highlighted the greater wear & tear on those batteries compared to his main experiences in Norway.
Manufacturers also prohibit the use of a portion of the battery, usually at the top few %, so you’ll see figures on my car battery of 50kWh capacity, but the real capacity is more like 45kWh usable, and that figure is further reduced when the battery is colder.
As we lease EVs on the Motability scheme, I’d suggest just charging to 100% the night before you need to use the car & not worrying too much about storing the battery at any particular capacity EXCEPT for a very low one. If you drive the Nissan Leaf then yes, try to prevent the battery from getting too hot, but the EVs with liquid cooling won’t be an issue, the battery never really being warm enough is more important as that means less range.
Note though that if you’re flying off & leaving the car parked for a few weeks, you may want to make sure the battery is charged enough to offset any discharge & phantom usage. But, it’ll be the 12V legacy lead acid battery that goes flat before the traction battery.