The leads/contacts are removed from the campaign. I believe you would see the history in the activity logs in Marketo, but no reference to the campaign the lead was deleted from
I encourage you to look also at task usage in your Salesforce storage usage. In our org, Marketo generated tasks, logged to each lead - Was sent email:, Clicked link in email:, etc. - accounts for the vast bulk of storage usage. I've estimated that each email blast to our house list generates 20Mb of data storage, in tasks, in Salesforce.
And the only answer I've had to the task storage issue is to regularly cull old tasks. i.e. what good does it do for the sales rep to know that two years ago, a prospect was sent an announcement for Software Release 1.5! ?