No, everybody doesn't know that. It's an allegation made by both parties when the numbers become inconvenient.
The basic BLS methodology hasn't changed in decades. In those rare cases where they've tweaked their methodology, they call it out and document it. For example, they revised demographics weighting in Bush 43's first term, creating a one-time ~900K increase in reported employment. (As someone who was not a Bush supporter, to put it mildly, I examined that change and had to concede it seemed legitimate, mostly reflecting a shift in the balance between farm and non-farm workers.)
The BLS has "always" maintained several sets of unemployment data, each using somewhat different criteria. The U3 rate is traditionally cited as the official unemployment rate. There are five others, however (from BLS):
- U-1, persons unemployed 15 weeks or longer, as a percent of the civilian labor force;
- U-2, job losers and persons who completed temporary jobs, as a percent of the civilian labor force;
- U-3, total unemployed, as a percent of the civilian labor force (this is the definition used for the official unemployment rate);
- U-4, total unemployed plus discouraged workers, as a percent of the civilian labor force plus discouraged workers;
- U-5, total unemployed, plus discouraged workers, plus all other marginally attached workers, as a percent of the civilian labor force plus all marginally attached workers; and
- U-6, total unemployed, plus all marginally attached workers, plus total employed part time for economic reasons, as a percent of the civilian labor force plus all marginally attached workers.
When the opposition party wants to cry about the unemployment rate being manipulated, they usually cite the higher U-6 rate instead. This has been the case for decades, whether that opposition party is Republican or Democratic. Right now the U-3 is still seen as Obama's U-3, so Trump and his supporters are crying about how it's artificially low. I predict that in a few months, when the U-3 is accepted as Trump's U-3, Donnie and the gang will be bragging about how low unemployment is under his reign.