They were meant to be single use. The "5 mph" part just means a 5 mph impact cannot damage lights, the engine, or anything related to the safety of the vehicle like brakes or fuel system. In 1980 they expanded the law to limit allowable damage to the bumper itself.
Even if the shocks are compressed because they have taken an impact and were not drilled and "pushed in" or "tucked" by someone for stylistic reasons, it's common for the moving parts to be scratched as they compress together, damaging the metal and potentially the seal. If one could successfully pull them back out, and the seal, oil, and nitrogen remained intact, the scraped portion is likely to rust resulting in impaired function in a subsequent impact. It's also common in cases where only one side compressed that the uncompressed side gets bent a bit and doesn't compress as easily if at all.
That being said, an original shock on a driver that's been aged and weathered may not function as intended, either.
If you just want it to look better, pull it out with a chain. If you want it to work as new, find some new shocks or have yours repaired and restored. I have no idea what the prospects are for either option.