You're reading too much into it, I seriously doubt they are setting El up to be a lesbian.
MANY girls at the time had rainbow in general and Rainbow Brite in particular shirts, posters, trapper keepers and sheets. In fact my wife had that exact bed sheet (the one with all of the rainbows). In the 80s, the rainbow wasn't 99% identified with everything gay. It was technically used as a gay pride symbol dating back to 1978 but by 1985 I don't think it was widespread outside of maybe San Fran. Out of curiousity, I put in gay pride parade 1985 and after the first four pages of Google images I didn't see a single rainbow flag. Just blue and red decorations in the American parades and a lot of pink (as it looks like the pink triangle was more commonly used at that time).
Not a single rainbow to be seen in 1985 gay pride parades.
And I realize the rainbow is now so commonly associated with gay pride that I can't even see a real life rainbow without being reminded of it, but at the time period Season 3 is set it didn't mean anything beyond "girly" to most people probably even most gay people.
They were going for girly not stinky boys in their 85 imagery when in the background for Max and El. These were the quintessential mid to late 80s tween and young teen straight girl icons.
Now those just look "gay” and no image conscious tween girl would be caught dead with the above as it seems like the modern tweens just dress like the modern teens and twentysomethings. But Lisa Frank and rainbow everything was “to die for” with the girls, tweens and early teens during that time.
And the “Bible” for Stranger Things back when it was called Montauk does not say that El will be gay, in fact it sets up that she is the romance for Mike. And almost everything in that Montauk “Series Bible” has been accurate EXCEPT that the conspiracy theorist didn’t show up until season two, the science teacher isn’t an “Indiana Joneseque rock star of science” but a simple nerd and they didn’t jump 10 years forward for season two and come back with the kids as young adults ala It. But all of the character and plot descriptions have been dead on and El is NOT a lesbian or bisexual.
Will on the other hand is 100% gay in the Montauk Bible. It’s only been “hinted at” (albeit very strongly with Mike this season saying “It’s not my fault you don’t like girls”) in the tv series but per the Bible Will is 100% gay.