Q.A Healthy Conifer Has Been Cut Back, Basically Leaving A 10ft Trunk
A neighbour says the trunk will now rot and is not safe, so I must remove it. Will the trunk rot or is it ok to leave just like other pollarded trees. Best Regards.

Certified GKH Gardening Expert
Conifers are not good candidates for pollarding. Your tree was topped, which is different. If no branches or foliage is left on the tree, it probably will not regrow. You can leave it for awhile, it will take some time to degrade.
https://www.gardeningknowhow.com/ornamental/trees/tgen/what-is-pollarding.htm
https://www.gardeningknowhow.com/ornamental/trees/tgen/pruning-coniferous-trees.htm