That’s indeed not valid since Facelets because it’s syntactically invalid in XML. You’d need to manually escape the XML special characters like <, > and so on.
<h:outputText value="<br/>" escape="false" />
You can however just emit the <br/> in template text without the need for a <h:outputText>.
<br/>
To render it conditionally, wrap it in for example a <ui:fragment>.
<ui:fragment rendered="#{bean.rendered}"><br /></ui:fragment>
A <h:panelGroup> is also valid as it doesn’t emit anything to the HTML anyway.
<h:panelGroup rendered="#{bean.rendered}"><br /></h:panelGroup>