Composition for Confluence requires an active app subscription, so uninstalling this information will be lost on the front end but available on the back end in the page’s storage format.
The body of the macro will not be lost if the plugin is not installed. It is Storage Format markup that is persisted by Confluence.
View the Storage Format for a page containing an HTML macro, and you can see that:
<ac:structured-macro ac:name="html-macro" ac:schema-version="1" data-layout="default" ac:macro-id="2be30073-6c34-41aa-9a35-bcc801e8eac8"><ac:plain-text-body><![CDATA[<b>Hello</b>]]></ac:plain-text-body></ac:structured-macro>