The omitted information must be predictable and easy to recover for the listener.
While it is a grammatical rule in some languages, in others like English, it is often associated with informal registers, such as diary entries , text messages, or casual spoken conversation (e.g., saying "Seen it" instead of "I have seen it"). Topic Drop Across Languages Drop the Low
The frequency and rules for dropping topics vary significantly between language types: Topic drop in German: Grammar and usage - ResearchGate The omitted information must be predictable and easy