I’ve tried downloading the 1.10.x .deb file from the repo, but it won’t install either, giving the same error: zerotier-one : Depends: libstdc++6 (>= 5) but 4.9.2-10+deb8u2 is installed
Will this be fixed, or am I forced to upgrade from Debian Jessie?
So, you’ve not been getting security updates for three years, the Debian maintainers moved on long ago, and other software providers moved to support more recent versions. So, yes, you will need to upgrade to something supported.