Television pilots are made every year and not all of them get picked up for a season or series. What happens to the pilot then? Does it get stored somewhere? Who holds the rights to it?
There is no standard practice for TV pilots (failed or otherwise), as their possessors will have different intentions. Some, but not all, successful pilots are deployed as the maiden episode of a TV series, with the rest of the show built off the back of it; others require recasting, and the pilot itself is buried or re-shot for consistency.
Failed pilots, however, typically have a much more varied existence.
Some are retained by their developing networks, in order to be used as proof of concept for another project (typically to showcase the talents of their respective writers).
Projects which deploy licensed characters are sometimes only created to stop the character rights lapsing. Wonder Woman is a possible recent example of this, as its inability to be picked up stunned people. It has, however, been used as leverage to produce Amazon by the same network: as if Wonder Woman needed a PoC, at this point!
Other works become cherished as lost treasures, in which case networks will release them to special events in order to be exhibited (Comic-Con/Memorabilia events)...
Lost pilots were, previously, scheduled into graveyard shifts in order to operate as novelties and pull an audience: this was allegedly back when there was a comparable lack of syndicated content to absorb some air time.
Basically, it's up to the developing studio: who in developing it will retain the rights to it. If a show wasn't picked up, ostensibly it's because it's not good enough (and hence an embarrassment to the studio). If this were the case, a Studio may wish to simply bury the pilot so it's not available for wide exhibition. It's entirely down to the mandate of the developers whether the show sees daylight.