Is it permissible to use a well that was dug for drinking purposes for other uses, such as a "siphon," after it has been abandoned, especially since some consider this forbidden?

If the well is private property, its owner may dispose of it after it is no longer needed, so long as there is no forbidden act or harm involved. However, if the well is an endowment (waqf) and its original purpose can no longer be served, it is permissible to sell it and use its price for a substitute endowment, or to dig a new well in a place of need. Alternatively, the beneficiary of the endowment may utilize it for any purpose other than drinking, because the intent of an endowment is benefit.