Embedded metal and tree felling
There is an ongoing debate about whether or not bolts should be used to support treehouses but normally this concern is only with the immediate health of the tree. A potentially hidden problem for the future of the tree is with embedded metal, as the fate of most residential trees is to be cut down because they are posing a danger or have outgrown their space.
Tree surgeons tend to have a dim view of treehouses, as many experience them when their chainsaw strikes a buried nail or bolt in the middle of a tree trunk. The usual suspects are fencing staples and wire, signs nailed to the trunk and old treehouses. Ancient and long-collapsed treehouses tend to leave collections of fasteners in rings which the tree grows around over time, hiding their locations. This means that when a tree is cut into sections there is a good chance a chainsaw blade will hit this embedded metal. Steel is damaging to any blade and it is easy to wreck expensive chains and chainsaw bars this way.
If the tree may be milled for lumber in the future long cuts will be impossible if any metal is in the path of the blade used to divide the trunk. Commercial timber yards use metal detectors to prevent trees with embedded metal from being fed into their saws, where the damage would be especially costly, but this system is not practical or economical for small scale felling operations.
There doesn't appear to be an ideal solution to this problem. Non-invasive methods such as slings over branches are often suggested, but the consensus amongst professional treehouse builders is to use small numbers of heavy duty bolts, as purely suspended supports carry the risk of cutting off the flow of sap under the bark, with a very real chance of limb or trunk death.
If you do use metal fasteners they are likely to remain in the tree for the rest of its life and this legacy should be considered at the planning stage, especially if the tree contains valuable timber that could be milled. Often bolts will be impossible to remove after a few years as the tree grows around them and it is easy for nails and screws to go unnoticed and be absorbed into the trunk. For now it seems the main advice is to keep a long-term record of the height of the support fixtures for your treehouse including photographs of the support positions. This can be passed on to future homeowners and may well prevent problems coming up years in the future. It is also important to reduce the amount of unidentified steel by removing any nails and screws used for temporary positioning once the supports have been bolted in place.