Good question!
It’s hard to imagine the Valar expressly banning any knowledge, even dangerous knowledge like sword making, but maybe certain trades or lines of inquiry became taboo amongst the Elves, even unlawful. Certainly the Valar wouldn’t stop the Elves from deciding amongst themselves that Melkorish Activities were no longer acceptable.
I’d give my left leg to know what information Tolkien thought Melkor was giving the Elves during that time; my guess is something related to ~the tainted realm of Science oOooOoOooO~ but what variety, I don’t know. They already had Aulë teaching them metalworking and mining and smelting etc., and the rest of the Valar were there to teach their specialties. Melkor’s contribution might have been to explain things in more depth than the other Valar. I could also see him giving more information about the Spring of Arda than his brethren, and spreading a less accepted telling of the Ainulindalë from his perspective– definitely biased, but not without truth. He might just have been teaching them crazy sex positions, who knows. And that is why double-reverse-upside-down-cowgirl is forbidden in Valinor to this day…
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