Re: Law in the UK is clearer.
"A legitimate use for a chainsaw equipped drone [,,,]"
I suspect a drone is too light to apply any real pressure to the saw to counteract the bounce from the blade's rotation.
An electric chainsaw on a very long pole would also do it - possibly. However there are two snags.
1) a chainsaw can get trapped in the cut and needs freeing manually.
2) when the branch is cut it can "hinge" rather than drop cleanly. There's a lot of weight in such a branch plus its momentum. It will damage anything that it hits - in this case the neighbour's fence.
Both these events happened recently when a tree surgeon was pruning a nearby tree.
As it is I will just have to hope that the ash tree gets taken out by the "die back" plague. That will stop its expanding canopy from blocking the afternoon sun in my garden.