I don't think I've ever heard staff say that you can't use a smiley (or similar) face on text mediums. I have heard them say that, based on the diverging timeline, 'emojis' never really became the huge cultural phenomenon they are today. 'Emojis' are modern real world pop culture so need to be treated as such. Same with hashtags.
Using smiley face on letters or on the grid or whatever is probably fine as long as you keep to the CyberPunk theme and don't try and put too much focus on this modern real world pop culture thing. I think that hashtags get less leeway though.
If I see a hoodie with a smiley face sticker on it, that sounds cool and themely. If I see someone in a kimono made of fabric with patterned 'emoji' print I think that doesn't make sense given the Sindome World. If I see a smiley face thrown into a grid post I think there's a smiley, whatever. If I see someone using hashtags or emoji eggplant come fuck me sentences, I think that's pop culture I'd rather not see in the game.
I see IC memes the same way. They are a huge modern pop culture thing that were not really a big thing when the timelines diverged.
Even if you could argue that 'maybe they ended up being developed in the Sindome timeline', I'd still not want to see memes or emoji use or hashtag use in any way approaching what I see IRL. That is my personal opinion.
Just my take on this topic based on feedback I've personally received. What is your take? What does staff have for feedback?