Those are the main social medias I think of where people would promote their videos on. I am in a couple of discord groups (not youtube related) and one of them has a self promotion section on the server, but again it is NOT a YouTube related discord server. And the other server actually has a no advertising rule on it, but they are aware that I have a channel because I have said before in the discord server like "I'm editing a video", or I might say like "I am at 5900 subscribers right now". It just can't appear that I am advertising in the server itself. However though, me mentioning my channel a bit without directly posting links does lead to people DMing me asking for the channel and that is when I give them my channel link.
I was explaining this to say that Discord could be used to promote your channel, but do it in the right ways and make sure to check the server rules before just directly promoting or advertising your channel there.
Also, not social media, but I have mentioned my channel to people at school when I meet new people after I have gotten to know them a bit and when they ask me what I do outside of school. I mention to them that I have a channel and what I make videos about, but I don't directly tell them my channel name unless they ask. It is still a way to promote your channel though because even if they don't ask for your channel name, it still gets the word out that you have a channel.
If you could work out a way to answer questions and then follow up with one of your videos to help demonstrate or explain the answer, then you could try out Quora. You just have to be careful with how you do it on Quora though because they don't want you to just flat out advertise your channel/videos there. Quora would be more for promoting single videos instead of your whole channel, but if you were able to include your video in the right way when answering a question then it could cause people searching for the answers to go to your channel from the video to see what else you have posted.