A former colleague can miss your company after the role has already been filled.

Filling a post and replacing a person are different operations. The work gets covered within weeks, and the particular way you made a difficult morning bearable does not, which is why people who barely spoke to you formally can find your absence surprisingly loud. The organisation is fine. The individuals in it are missing something the organisation never recorded.

If you have left somewhere, one message to a former colleague costs nothing and often lands better than you expect. And if someone has gone from your own place, tell them what you miss specifically rather than assuming they know.