'forever' is a big word to be bandying about. However I'd think that if there were some way to grow a spring as a single metalic crystal the way jet turbine blades are you'd come very close. However you need to balance the extreme cost of cost a process with what is almost certainly a lesser cost of regular maintenance and replacement. For example nobody wants to...
I can't help wondering what it would be used for - perhaps a Jack-in-the-Box would be fun, to surprise archaeologists a couple of thousand years from now! ;-)
At a pure practical level I have found all springs weaken over a period of time whether under compression or tension. It does of course depend on the conditions.
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