Wow, that is a new one and I can assure you 100% that what you heard is just that, A RUMOR. Heck, that does not even make any sense when you think about it. If you made hooks out of lead or a mix with lead your hook would be softer than a paper clip.
Perhaps that rumor came from the fact that several years back when CA instituted their lead laws and changed the wording every few weeks (leaving companies open to a lawsuit if incorrect lead warning was used), Mustad discontinued some of the bass hooks for soft plastics that had lead weights on them. Newer versions were reintroduced a couple years later, once the lead warning label text was finaly settled upon. Had nothing to do with the hook wire or fly hooks for that matter.
Mustad is THE ONLY hook manufacturer in the world who uses wire specifically formulated for making hooks (our own secret formula) and the only one who draws our our own wire to get the diameters we need.
It's always interesting to hear what's floating around the rumor mill . . .
in any event, carry on, it's an interesting thread . . . : )