https://en.wikipedia.org/wiki/United_States_Forest_Service
The department has existed at least as a management agency for 150 years. How long has the department have timber cruisers out running around in virgin, old growth forests in primary Sasquatch habitat in the Pacific Northwest where they would run across sasquatch footprints, howls, sightings, etc? The earliest references to "rangers" and government timber sales surveys are right about the turn of the 20th century (or about 120 years). But there is an even more critical example of government knowing about sasquatchery:
https://en.wikipedia.org/wiki/Fort_Lewis_(Washington)
From the beginning of the existence of Ft. Lewis (109 years ago), situated in prime sasquatch territory and in Pierce County, WA, which features one of the highest densities of sasquatch reports in the nation, there have been soldiers (thousands of them) running around, camping, conducting nighttime training, and generally living in the woods there. There is absolutely, positively no way (no how) that sasquatch reports have not been regularly reported to the post commanders (and thus to commanding generals, almost certainly "unofficially") over the past 109 years.
If sasquatches exist (or existed), Pierce County WA was one of the most densely inhabited areas of their range (according to report densities), and the Army knew it. Guaranteed.
The Army operates (and always has, since before the founding of our nation and when it was under the Union Jack) under a "need to know" basis. It's institutional. No, every soldier in the Army, or even every soldier who was posted at Ft. Lewis doesn't know, so the "conspiracy" isn't "giant".
I know of few to no reports of thunderbirds, and such reports would likely be most interesting to the USFWS, not the USFS. Currently, there is quite a big issue with giant snakes in Florida, and the reason why the USFS is not directly involved with it is because the USFS isn't managing forests there, but the USFWS and NPS are deeply involved, and there is no secrecy because every agency involved (both state and federal) have determined that killing the snakes are in the best interests of everybody, so they encourage that. If the USFS and Army determined that the killing of sasquatches was desired, I can assure you that they'd let you know about them..........and my suspicion is that they keep their existence completely unofficial specifically in order to discourage the harassment, disturbing, hunting, bothering, and killing of sasquatches to a minimum. The best way to do that with the public is to keep them a myth. That was incredibly easy to do prior to 1967. The PGF made it a bit more difficult. So, essentially, sasquatches have been publicly "out-of-the-bag" for 59 years.
One doesn't keep secrets by recording them on paper..........or now in emails, even if you hide them in your bathroom at home. These kinds of secrets are word of mouth, and kept among few trusted persons.