Preact - how to display error page when path is incorrect.

I think, this article answers for your question.

Use path="/" and default={true} props with error page in <Router>.

Example: 

<Router>
    <Page1Component path="/page-1" />
    <Page2Component path="/page-2" />
    <Page3Component path="/page-3" />

    <ErrorPageComponent default={true} />
</Router>

References

1. Preact - preact-router default routing