Well, support for Win 10 doesn’t end until Oct., 2025, so you have awhile, but whether new SW & HW products will continue to port to Win 10 depends on the specific producer. Whether you choose to upgrade or not, Any new machine you purchase will likely be Win 11. Honestly, it’s not that different from 10 - maybe a bit faster, some new features, and it is very stable. If you’re building or buying a new machine, it makes sense - if not, no rush, and many Win 10 machines do not include trusted platform support so can’t be easily upgraded without a work-around/hack.

The ban on lithium batteries in checked luggage has been in place for a long time. If that's what the warning is about then it's nothing new.

And there's a limit on the size (capacity) of lithium ion batteries you can carry on, but I don't think any camera battery would exceed that capacity. Here's a link to the TSA's description of what you can take https://www.tsa.gov/travel/security-screening/whatcanibring/items/lithium-batteries-more-100-watt-hours.

I have routinely carried a 26,000 mAH power bank in my carry on computer bag for years without any interest from the security screeners - I've read that this rating is just slightly below the 100 watt hour limit. The numbers I could find quickly for the EN-EL15c battery (made specifically for the Z6ii camera) are 2,280 mAH/16.4 WH, and the other EL15 versions are probably a bit lower. So they should be fine in carry on bags.