Can a guy and a girl be just friends after they tried to date?

Me and my ex were best friends all through high school and once we finished high school we thought we owed it to ourselves to at least try and date. It didn't turn into anything serious and it hardly even lasted a few months. He was my first boyfriend, my first kiss, and i can't even consider him my 'ex' because the relationship was literally nothing that serious at all (i would sleep over in the same bed but all we would do was watch movies and eat food). I think he had (and still has) some interest in me but a lot of it is just best-friend feelings.

After we 'tried' to date we lost contact for a few months because we both were really busy with our jobs and personal life and i think it was partially due to the fact that I was worried that if i messaged him too much it would come across as if I was 'flirting'. I get so worried thinking that if he has a girlfriend, my messaging him 'Hey how are you going!' would be considered inappropriate. Im not being flirty i just GENUINELY care and want to know how his week has been so is it okay for me to just randomly message him even if he has a girlfriend? If i was a guy it would be fine but is it okay for a friend who is a girl to do that? If not WHY!

Why is there all this societal stigma over a boy and a girl just being 'friends'. Why do people wink at you and shoot you sarcastic looks as if they don't believe you when you reply with 'we are just friends'?

My question is: Is it normal for a girl and a boy who once tried to date to still be friends? Don't just straight away say 'of course why wouldnt it be' because honestly so many girls i know hate their boyfriends having friends who are girls let alone ones who they DATED.

I invited him to come to a concert with me after months of no contact, i didnt even know if he has a girlfriend ( i just found out he doesnt) but should it matter? Im not asking him on a date and ive made that perfectly clear. He was the first person i thought of when i bought the tickets, no romantic feelings just pure 'i want to take this friend with me' so why do i feel like it needs to have a label?