There's no evidence the text actually exists, though. There are images of it but I don't see the actual message file etc being posted anywhere so even it's mere presence is an assumption based on questionable evidence. The image metadata was intentionally and clumsily forged which tarnishes the validity of anything else associated with it.
There are a surprising number of large aircraft disappearances, almost universally over water. Since 1970:
13th June 1971: USAF RC-135 (military reconnaissance version of the Boeing 707 4 engine narrow body airliner) disappears off the coast of Hawaii during monitoring of atmospheric nuclear tests. Only small debris ever found.
21st April 1981- Spanish C-47 cargo aircraft (military Douglas DC-3 variant) vanishes over the Mediterranean.
13th February 1983- Learjet 35 (privately owned long-range business jet) vanishes over the Straits of Malacca. No wreckage ever found.
25th March 1986- Indian Air Force Antonov An-32 transport aircraft vanishes without a trace during its delivery flight from the Soviet Union.
25th August 1989- Pakistan Airlines Flight 404, a Fokker 27 Friendship with 54 passengers on board disappears over the Himalayas. Wreckage never found.
22nd December 1997- an An-72 cargo transport disappears between Ivory Coast and Namibia. Wreckage never found.
May 25th 2003- a Boeing 727-233 is stolen from an airport in Angola and disappears, likely somewhere over the Atlantic.
There are numerous other unexplained disappearances of aircraft that predate these as well as several incidents where aircraft disappear without a trace (Star Ariel, Star Dust, Star Tiger), or in unexplained circumstances, for a number of years before the mystery is later solved. The Air France flight (447?) which vanished off the coast of Brazil with only the vertical stabiliser and some small wreckage and a few bodies found had similar conspiracy theories about it and that turned out to be an aerodynamic stall caused by pitot tubes being blocked with ice.
Uruguayan Air Force flight 571 was only found because the survivors marched ten days across the mountains back to civilisation, and it still took 72 days. The Uruguayan authorities had given up hunting for it weeks before that point.
Pretty sure there are others.
That's crazy, I had no idea black boxes were so ineffective. Why don't they simply stream all the information from a blackbox to control center? It seems that'd save quite a bit of trouble having to search for them.