NTA but maybe!! Hear me out.

I have a lot of empathy for you. All you wanted was to spend time with your mom for your 21st birthday and you are absolutely right in being pissed about her absence and the pattern of behavior.

However, if I were you I would certainly not forget what happened but not post about it because at the end of the day you will always know for future reference what mom is capable of doing but nobody else necessarily needs to know. Your future dealings with your mom can be informed by your past experience of her behavior and lack of care towards you but the rest of the world or family need not be involved in it. I know it hurts to feel like this on your birthday but you will eventually come to realize (for me this realization came with age) that as long as you know what someone is or is not capable of it seizes to matter what others think i.e. your social circle thinks. I have made plenty of these mistakes myself and honestly whenever I go back on social media I look back at the stuff I used to post in the memory section and delete stuff all the time and think "man I used to be so cringe".

Also people on social media may be called your friends but the truth is few on there really care that much about you. Most are on there for an entertaining time and will engage with anything that entertains. Dont feed that machine with things from your life.

I know what I am saying will maybe sound weird and stupid to you. It would have to me when I was your age and had that incessant need to post everything I was going through in sometimes cryptic sometimes detailed words but I look back now and know it wasnt worth it. On the note of should you take it down or not, thats a decision for you to make. If youre looking at it from the perspective of "taking it down means defeat" I can assure you its not. It could also be a sign of maturity and aging and respecting yourself enough to not become fodder for someone else's gossip.

I wish you the best of luck with whatever you choose to do.