Mm

or anyone else, since you search old rules and books (at first i read 1998 and i thought, aaa this is not
that old

), you mentioned before who introduced the music in programs, is there any idea how the rule about no lyrics in music pieces came up and how /why eventually they changed it for ice dance?...
I believe that for a long time there was no actual rule about vocal music, but it was just a tradition to use instrumental music only. The first musical accompaniment was just the town band, hired to stand on the side of the lake and oom-pah away. Later, when indoor rinks were built for recreational skating, there would be an organ, the same as for accompanying silent movies in the movie houses.
(OT -- William Hershel (organist and oboist) and his sister Caroline (soprano) entertained between acts at a famous London theater in the 1700s. When the next act of the play commenced they had some time off before the next interlude. So they rushed home to look through their telescope until it was time to run back to the theater again.
They discovered the planet Uranus (which they named George (after the King of England), but Continental astronomers didn't like this name, so they changed it..
This is true -- you can look it up.

)
I am not sure when major figure skating championships started to be held indoors, but I have seen photos of skaters in the 30s and 40s with phonographs at the edge of (outdoor) rinks.
I am pretty sure that in the late 90s some pairs team (Kuchikia and Sand for one) skated to opera music with vocals. But as for popular music, there were a lot of rules in place about "not making serious competitive skating into a public spectacle." Things like costume rules, rules against show-off elements like backflips and head-bangers -- and it was generally expected that skaters would use sedate classical symphonic-type music. I believe that the ISU passed a specific rule against vocals of all types after the 1989-90 season.
If I remember correctly the thin edge of the wedge came in the 1997-98 Olympic year. The Original Dance was the jive, and the skaters had a hard time finding appropriate music with no vocals. So the ISU relaxed the rule for ice dancing that year. I don't know if they allowed only "scat" vocals that year or full vocals for the OD.
But basically I think the impetus on disallowing vocals was to keep the pristine stick-up-the-behind image of serious competitive skating untarnished by anything connected to professional show skating.
(Caveat: Skating experts are welcome to correct any of this if I am remembering it wrong.)