That's not a principle. That's just a random thing that people thought back in the 80s and 90s because of some clever little comment in a spec, that sounded correct at the time but proved absolutely ruinous in the decades since.

Systems that are liberal in what they accept are paradoxically harder to develop against, undermine standards and encourage incorrect, fragile implementations. Query parameters are a classic. In practice, unrecognized query parameters almost always represent a bug in the client. Better to find out immediately with a 400.