음-form at end of sentence It is more unnatural than informal. No one really speaks like this in a real conversation because it sounds awful. It is mostly used in an informal and space-constrained writing (like online chats), or for jotting down things for oneself. In short, it is a jarring, aesthetically bad style that has a limited use in the writing form only. Another reason it is used online is because people sometimes want to avoid the normal sentence endings. Normal endings have either the plain or polite form (except some question endings like -는가?, -나?), but sometimes neither sounds right. The plain form would sound rude and the polite form would sound old-fashioned and fussy (and longer too). So people go for this -ㅁ/음 form which is neither of these and at the same time saves space and time.

돌아갔어 > past tense or present perfect. 저 돌아감 = Imma out.