I know you can't do much about it from the side coming in to replace someone, but waiting until someone is on the way out is fundamentally a flawed way to do it. As an employer you need to be focused on seamless onboarding _before_ the people who know how to do things have decided to leave. "Knowledge transfer" is just new-hire onboarding done by someone who no longer has a stake in seeing it succeed.

I'm not sure how the agency is structured - but what I can vouch for (and seen work) is continuous communication amongst coworkers in the form of tech talks and documentation. That way, knowledge is distributed within the team and a situation like this is avoided upfront.

I'm reminded of the Bus Factor concept: https://en.wikipedia.org/wiki/Bus_factor

Most companies I've worked in had an onboarding checklist or guide for new devs. You should also have an offboarding checklist. Whenever you add something the onboarding checklist, add it to the offboarding checklist as well ("grant Jira access to project X" => "revoke Jira access to project X" etc)