Tbh means "to be honest" it's not an Instagram thing but a text term/anonym gramps... For iG it's throwback thursday = #tbt... That wouldn't have even worked if you read the context or the fact that there's no hashtag or even different letters....
Tbh isn't an Instagram thing unless you just use the text shortcut for to be honest in your text
You wrote tbt.
Tbh was around BEFORE ig. Same with tbt.
Your phone (or brain) wrote tbt instead of tbh.
I was making a joke. Sorry I didn't write "lol jk."
Ffs I grew up in the 90's bro, we were the fuckin pioneers of lazy Internet slang.
Along with that, if you start using hashtags on IM, I will have you exiled to bop. Or ntbm.