I see this as addressing a symptom rather than the cause.

A direct result of technical staff leaving is the loss of siloed knowledge, instead of trying to address it at the final juncture, my opinion is that this type of knowledge should be proactively documented and shared with the rest of the team as the domain expert learns/works on it. That way, they're around to mentor other members, reduces the bus factor and when said person leaves, the rest of the team naturally continues their work rather than spending time on ramp up tasks.