In "Return of the Jedi", Mon Mothma briefed the Rebellion leaders on the plan to assault the Death Star II, and famously said the following line:
Many Bothans died to bring us this information.
In Legends continuity, it was established that the Bothan deaths occurred when Vader single-handedly slaughtered an entire intelligence cell of Bothan operatives while attempting to retrieve the plans. However, when the canon changed, both that event as well as the prior events which lead to the Bothans having the information was ret-conned out of existence.
Therefore, in the Disney canon, what was the Bothan involvement in the Rebellion's plans, and how did their deaths come about?