Natural extinction; something (a relationship, system, etc.) coming to an end over time without deliberate termination.
古い規則は長年の放置で自然消滅してしまった。
The old rule disappeared over the years through neglect.
契約は更新されずに自然消滅した。
The contract naturally lapsed without renewal.
Ghosting: the unexplained, sudden end of all communication and contact with another person (often used for romantic relationships).
彼とは最近連絡が途絶え、事実上自然消滅したようだ。
I haven't heard from him recently; it seems the relationship has effectively faded away.
SNSでの出会いは自然消滅しやすいと言われる。
It's said that relationships begun on social media tend to fade away easily.
彼女に突然連絡が来なくなって、関係は自然消滅した。
She suddenly stopped contacting me, and the relationship faded away.