When a user account is deleted, all information that is private to that user is removed and all shared records remain unchanged.
Before deleting the user account you can check for private records by: resetting the account’s password > login to their account > set the search filter to “Show private” for contacts, companies and project blogs. If there are records you want to keep, share those private records with a workgroup by clicking on the checkbox in the list header > choosing “change permissions” > choose a workgroup to move them to.
You can prevent users from saving private records in the first place, forcing them to share all data with a workgroup, by choosing the option under Solve menu > Modify user accounts > Prevent private data.
When a deleted user is mentioned on shared records (in places such as assigned-to, responsible, attendees and meta data like created, last updated, completed by), their name will continue to be universally shown on those records.