Here are relevant P99 threads about this song and adjustments that have been made to it over the years:
https://www.project1999.com/forums/s...ad.php?t=52422
https://www.project1999.com/forums/s...d.php?t=100276
Based on the evidence vis a vis Live, original nerf to Lullaby happened in August 2000 (per the Abashi post in the whineplay thread). At that point it was changed as above, to get progressively worse and worse as the bard gets higher in level, until in the ~40s when it's more or less useless even on greens. I don't believe there is a hard level cap.
Lots of posts well into 2001 talking about being unable for example to mez the EC orc camps as they'd all resist (primary + secondary resists resulted in something like a ~70-75% resist rate at higher levels even on low greens):
https://web.archive.org/web/20021028...gs.asp?song=17
Before the August 2000 nerf though, Lullaby was indeed very OP and bards were the premier mez class in groups.