A new conditional substitution {if: expired} lets you insert a message to people whose membership renewal is overdue, for example, to remind them.
When you use this, you would probably also want to include a personalised link to the renewal form. This is similar to the renewal reminder email. (A renewal reminder doesn’t need a conditional substitution, though, because the recipients are, by definition, overdue).
For example:
{if: expired}Your membership has expired. Renew here
Set the renewal link to refer to the page on your website which embeds the renewal form. Then use the personalise link button in the editor toolbar. That makes the link personal, so the form already knows who they are.