I remember back in high school being excoriated by an English teacher who said, in effect, "You can’t say a myriad of. Myriad is an adjective."
So, blindly, I believed that for the next several decades until…today.
I finally looked it up. Turns out myriad started out as a noun meaning "innumerable" or literally "10,000" (once considered an "innumerable" sum).
Then in the 19th century, Samuel Taylor Coleridge helped convert it into an adjective when he wrote the phrase, "Myriad of myriad lives."
Not sure what that meant or referred to, but as a result, we now have myriad as both noun and adjective to abuse.