This (admittedly naive) question (asking whether the existence of tachyons would have observable consequences) seems to me to be clearly about mainstream physics, though it was closed as non-mainstream.
If the answer is that the existence of tachyons would have observable consequences that contradict mainstream physics, that answer is mainstream physics.
Mainstream textbooks often say things like "Perpetual motion machines are impossible because they would have such-and-such observable consequences which contradict other things we know about". Why can't a mainstream Q&A treat tachyons the same way?