First decide whether the domain should continue receiving mail during a transition period.
Inventory active mailboxes, aliases, applications, SaaS senders, certificates and external accounts that use the domain.
Remove sending dependencies, then clean up SPF/DKIM/DMARC and provider verification records when they are no longer required.
If the domain should never receive email, a Null MX can explicitly signal that state.
Keep ownership of a retired business domain as long as the organization needs to prevent somebody else from acquiring it and receiving traffic intended for old addresses.