Things which are signs society devalues friendship:
1. It is considered normal to move a long distance for the sake of family or romantic partners. It is not considered normal to move a long distance for the sake of friends.
2. You can get compassionate leave from work to care for a sick partner or family member but not for a friend (even if the friend lives with you.) There is sometimes an exception if you are a Registered Carer.
3. People will commiserate you far more over a break up than over the end of a friendship.
4. The whole “men and women can’t be friends” nonsense.
But I don’t think slash fiction is a sign society devalues friendship. I think slash fiction is a sign that a small, not especially powerful section of society values Representation and/or Hot Boys Kissing (probably more of the latter tbh, otherwise there would be more femslash and more poc-centric fanfiction.) Nobody is saying “Steve has to kiss Bucky because nobody cares that much about measley old FRIENDS. They’re saying” “Steve has to kiss Bucky because Bisexual Steve Rogers is important for Social Justice” and/or “Steve has to kiss Bucky because fap fap fap”
Idk, I think slash might be part of a larger picture in which society devalues friendship. My roommate (also straight) and I have a pretty close relationship, and I’ve had people (both guys and girls, both straight and not) tell me that they ship the two of us. I think that’s pretty clearly motivated not by desire for Representation or Hot Girls Kissing but by this sense that platonic relationships are inherently less interesting/complex/high-stakes/intimate than sexual relationships.
Maybe that’s not exactly the same thing as slash but like, I do think a lot of people I know have this tendency to respond “I ship it” to any relationship between characters or real people they find interesting, even if I think it’s more interesting when viewed as a friendship.