as to the OP, i'm not particularly old, but i'm aware of some of the myths that were popular at various times... some still are!
there are lots of myths about ways to get more sustain including: heavier basses (the T-40 was deliberatly made to be that heavy because Hartley Peavey was convinced it gave better sustain), string-thru-body basses (there's a concurrent thread on this as we type), neck-thru probably has some factual basis, but in general it still seems like more of an old-wives tale than anything else. I own both neck through and bolt on, and they seem to have a slightly different feel more than anything, but the difference in sustain is pretty negligible.
actually the string-boiling thing is a myth too according to a lot of folks. YES, it will clean the crud out, but it also causes the strings to slowly rust from the inside, or at least that's what lots of thread-folks have said.
a real old one was that the pickup covers on old Fenders were meant to reduce hum, which they didn't do at all.
some would say the "sonic differences" between maple and rosewood fingerboards are a myth, but that's another can of worms entirely, and folks will argue to the death for both views. and you can add the possible "myth of tone woods" into this category. i'm not making a statement on it!
i'm sure there are more, but that's all i had off the top of my head.