Deleting a team member will not cause any impact to feature flags or other shared resources that the member created. The names of the deleted members will remain in the audit log even after they have been removed. Flags that the member maintained will have their maintainer set to null.
If the team member has any personal access tokens, those will no longer work. Personal access tokens should not be directly used in your code, as they are intended only for personal use so this is unlikely to have an impact.