My company usually designates a temporary internal 'replacement' to take over a former employee's accounts (opportunities, email) so any notifications sent to FormerWorker will forward automatically (via IT's designation in Outlook) to TemporaryWorker. My Salesforce team doesn't reassign the accounts to TemporaryWoker, they wait until a NewHire is selected. Then when the NewHire starts, they'll start receiving the emails instead (assuming that you're using alerts for "Sales Owner".
If you're using alerts based on a specific email address, like maybe including the digital marketing manager on certain notifications, then I have all of that documented in another system (Confluence, shared docs of some sort, etc) with key words that make it easy for me to find the information and direct links to the smart campaigns to take the guesswork out of it.
For some very custom things, I not only document in a shared system, I also put it into a special naming convention in the name of the smart campaign so I can easily search for it.
In most cases I've seen, the old salesperson's email is forwarded to a designated replacement (sales leader, another salesperson, etc) as a safety net to make sure nothing is missed - alerts or actual emails from prospects. Then the SFDC admin pulls a list of the salesperson's accounts/leads and a sales leader decides how they should be reassigned among the existing team and/or with a replacement hire. Then the clean up typically happens on the SFDC side, reassigning leads and accounts to the new owners.