When an academic would accept a connect request on Linkedin?

I read that some academics would avoid accepting students(undergraduate or graduate) or strangers requests on linkedin. So what are the cases that could be acceptable for academics to add people on linkedin?

• After completing their course?


• After working on a joint research?


• After attending a workshop, seminar, talk?


• After meeting them at a conference?


• After sending them an email about mutual work, papers etc?