most emoji are banned, as are nonstandard space symbols like thin space and some other things. emoticons (things like :-) and XD) are allowed. generally alphabetic characters and digits are allowed, and some other things, but i think unknown unicode is forbidden by default. https://news.ycombinator.com/item?id=23440551 has some investigation and results which are probably still accurate

i do most of my algebraic formulas with the compose key and a custom compose map mostly written by mark shoulson https://github.com/kragen/xcompose

for the above, after a false start picking random unicode characters, i realized that maybe i shouldn't use a modern language because someone who actually uses the language might feel like i was calling them an extraterrestrial, so i switched to googling archaic scripts. i pasted part of the old permic table from wikipedia

    >>> s = '''𐍐‎    𐍑‎       𐍒‎       𐍓‎       𐍔‎       𐍕‎       𐍖‎       𐍗‎       𐍘‎       𐍙‎       𐍚‎       𐍛‎       𐍜‎       𐍝‎       𐍞‎       𐍟‎
    ... U+1036x     𐍠‎       𐍡‎       𐍢‎       𐍣‎       𐍤‎       𐍥‎       𐍦‎       𐍧‎       𐍨‎       𐍩‎       𐍪‎       𐍫‎       𐍬‎       𐍭‎       𐍮‎       𐍯‎
    ... U+1037x     𐍰‎       𐍱‎       𐍲‎       𐍳‎       𐍴‎       𐍵‎'''
    >>> print(''.join(c for c in s if ord(c) >= 0x10350))
    𐍐𐍑𐍒𐍓𐍔𐍕𐍖𐍗𐍘𐍙𐍚𐍛𐍜𐍝𐍞𐍟𐍠𐍡𐍢𐍣𐍤𐍥𐍦𐍧𐍨𐍩𐍪𐍫𐍬𐍭𐍮𐍯𐍰𐍱𐍲𐍳𐍴𐍵
    >>> len(''.join(c for c in s if ord(c) >= 0x10350))
    38
    >>> permic = (''.join(c for c in s if ord(c) >= 0x10350))
    >>> ''.join(random.choice(permic) for i in range(5))
    '𐍨𐍓𐍰𐍚𐍠'

and then i tried out the hieroglyphs range

    >>> print(''.join(chr(i) for i in range(0x13000, 0x14000)))
    𓀀𓀁𓀂𓀃𓀄𓀅𓀆𓀇𓀈𓀉𓀊𓀋𓀌𓀍𓀎𓀏𓀐𓀑𓀒𓀓𓀔𓀕𓀖𓀗𓀘𓀙𓀚𓀛𓀜𓀝𓀞𓀟𓀠𓀡𓀢𓀣𓀤𓀥𓀦𓀧𓀨𓀩𓀪𓀫𓀬𓀭...
    𓿦𓿧𓿨𓿩𓿪𓿫𓿬𓿭𓿮𓿯𓿰𓿱𓿲𓿳𓿴𓿵𓿶𓿷𓿸𓿹𓿺𓿻𓿼𓿽𓿾𓿿

but realized that most of them were unassigned, at least in my font and probably in the current unicode standard (in case someone discovers a new hieroglyph), so i just did this

    >>> hiero = (''.join(chr(i) for i in range(0x13000, 0x13100)))
    >>> ''.join(random.choice(hiero) for i in range(4))
    '𓃾𓂃𓂌𓀩'

you can do all this in python in termux on your phone too (you'll probably have to install it from f-droid) but it's a bit clumsier

it's funny how this conversation has swung from the extreme of universal constants of the universe to the opposite extreme of completely arbitrary and historically contingent things like which ideograms (themselves completely arbitrary) are prevented from being posted by implementation bugs in hacker news