Well Known/Commonly used:

/j - joke/joking
/hj - half joke/joking
/s or /sar - sarcasm
/srs - serious
/hsrs - half serious
/nsrs - not serious
/p or /pla - platonic
/r - romantic
/sx or /x - sexual/sexual intent
/nsx or /nx - non-sexual/no sexual intent
/th - threat
/pos - positive
/neu - neutral
/neg - negative
/nm - not mad
/nbh - nobody here
/sbh - somebody here
/lyr - lyrics
/ref - reference
/t - teasing
/hyp - hyperbole
/li - literally
/ij - inside joke
/rh, /rht, or /rhe - rhetorical
/m - metaphorical/metaphorically
/g or /gen - genuine
/gq or /genq - genuine question
/c - copypasta
/cb - clickbait
/ncb - not clickbait

Not as common:

/nam - not actually mad
/ot - off-topic
/nav - not a vent/ not venting
/npa - not passive aggressive
/pa - passive aggressive
/nbr - not being rude
/nb - not bad/not a bad thing
/nay - not at you
/ay - at you
/lu - a little upset
/info - information/info
/nw - not weird
/hm or /sm - sorta/half mad
/vm - very mad
/np - not patronizing
/nay - not at you
/ay - at you

System Specific:

/iw - inner world/headspace
/ow - outerworld/body/irl
/isys - in system
/osys - not in system/out of system
/st - source talk