Those are official abbreviations that can be found in a dictionary.
The ones OP posted aren't all official.
TBH and SMH are official.
IMO is internet slang.
There's not a lot of consensus on internet slang abbreviations in Danish. It was more common back in the texting days, when all girls would end their messages with an S for "smiling", SS for "smiling sweetly", or KKK for "hugs hugs and kisses".
Question from a curious mountainmonkey: We have the same phrases and abbreviations up here, and is Danish equally as subject to people abusing "I forhold til" when they actually mean "I forbindelse med" or "med tanke på" ?
Yes, they're abused and mixed up. Best writing practice is to avoid them, because these phrases can often be left out in the first place or at least written more clearly.