In the era before VCRs and cable television, many promotions (from tiny local promotions to those covering entire regions of the country) had their own syndicated programs. Until the late 1970s/early 1980s, videotape (for those promotions that could afford it) was an expensive commodity, meaning that once a show aired it was reused for taping a future week's show. That, plus the perceived lack of future interest in classic wrestling programs (some stations destroyed the films once aired) probably means that, except for those who are lucky enough to have preserved videotapes/films of the programs, the weekly syndicated programs of many promotions — especially the smaller ones — are forever lost to history.
That said, World Wrestling Entertainment has hundreds of thousands of hours of videotapes and films of classic matches, including its classic syndicated and cable programs (at least from the early-1980s forward), plus programs and matches from now-defunct rivals including the NWA(specifically JCP, the NWA still exists and has its own library on demand), WCW, ECW and the AWA.
Some larger independent promotions have syndicated shows that are only shown regionally, and may only be clip shows. Usually this isn't a big deal, but occasionally a really good match takes place, and with the exception of the live audience is never seen in full by anyone. (Usually, the matches are edited for time, placement of commercials and to remove "dull" moments.)
Arguably the best example of one of these is an ECW match between Yoshihiro Tajiri and Psicosis. For some background info - Psicosis was running out his WCW contract and planning to go to ECW, and the mutants were very high on the possible matches. Psicosis and Tajiri outperformed even the mutants' expectations and put on a near-5star match. But... it aired on ECW's syndicated show instead of ECW on TNN, and the whole thing to date hasn't been seen, though the match can be found on the Extreme Championship Wrestling: Deep Impact DVD.
Rey Mysterio Jr.'s WCW career from the point he was unmasked to the demise of the promotion will never be mentioned or marketed by WWE because of the fact that they want to keep his face a mystery. He does however mention the subject in his book, advising fans to check out his unmasked footage online if it interests them.
Dragon Gate has a weekly television show, but they're only allotted one hour. So they record the show and then "clip" the matches (and occasionally omit some less important ones) to fit the TV show. The full matches are never seen by anyone aside from the live fans.
Several title changes of the WWF's major titles have never been recorded — or if they were, were never shown on television. Two prime examples came early in the history of the Intercontinental Championship:
Pedro Morales winning the title from arrogant weightlifting hero Ken Patera. The match took place December 8, 1980 — the night of the murder of John Lennon — at New York's Madison Square Garden. It is unclear whether that night's card was televised (as the venue's cards routinely were through 1992) on New York's MSG channel, but if it was the broadcast would have been pre-empted due to breaking news coverage of Lennon's death.
Tito Santana defeating the evil Magnificent Muraco on February 11, 1984, for the title. The most common explanation for the lost footage was that, in taping matches for later broadcast, there was no more videotape available to tape the Santana-Muraco match. Only brief clips from the match exist today and were later included as part of several video releases showcasing the title's history.
Over The Edge 1999 is one of the few PPVs to not have appeared on video or repeat. This is because Owen Hart died whilst he was making his entrance from the ceiling. The footage of Owen falling exists in the vaults but on the PPV itself it was replaced by a different camera facing the crowd. Owen's wife filed a lawsuit against WWE to ensure they would never release the footage, so it is unlikely we'll see it. Many fans nevertheless want to see it out of morbid curiosity. Photos from Owen being attended to in the ring do exist though.
The WWE Network, the company's streaming on-demand service, features an edited version of Over The Edge. All references to Owen, his accident, and his death have been removed, with the exception of a newly-added photo dedication to him at the beginning.
A number of WCW Nitro episodes were not aired by WWE's On Demand & 24/7 services, because of technical issues with the master tapes, or (in some cases) because of multiple appearances by Chris Benoit.
There are plenty of dark matches that are never recorded, but there are also some of these that have only happened once, between famous workers. Possibly the most infamous among the IWC is a match between Bryan Danielson and Minoru Suzuki on June 20, 2004, which was the only one-on-one match the two ever had and no footage of it is known to exist.
Another such example that was intended to be filmed was an IWA-MS match between Eddie Guerrero and CM Punk that was prevented by WWE refusing to allow it to be filmed.
The 25 June 2007 episode of WWE Monday Night Raw is available on WWE Network...but not as originally broadcast. The original edition was a three-hour empty arena tribute to Chris Benoit that was aired live in the U.S. & Canada. Unfortunately, the nature of the events which lead to Benoit's death were only confirmed during the live broadcast, forcing WWE to completely scrap the episode from international distribution and replace it with an in-studio recap of championship matches from earlier in the year. The original episode does not appear on the streaming service, and likely never will.