Hi there,
I’d like to hide the bottom bar because my website is already set up to work as PWA itself. I read it’s mandatory to use it (true?). So, is it possible to hide it?
Thank you.
Is this website still working? 
Yo tengo la misma duda que tu, he mandado un correo electronico con la pregunta, si me contestan te digo
Esto me han contestado el equipo tecnico.
Para ocultar la barra inferior dentro de la aplicación, puede seguir estos pasos:
Vaya a la página Apariencia > Barra inferior.
Habilite solo un botón de las opciones disponibles.
Al tener solo un botón habilitado, la barra inferior no se mostrará en la aplicación.
Yes, it is possible to hide the bottom bar. To do this you have to enable only one bottom bar item in Appearance → Bottom Bar, and the item type of that item will be the landing screen of your application (i.e. the first screen that will be shown to your users before or after login depending on your Settings → Users preferences). Even though you are setting one bottom bar item you will not be seeing the bottom bar when you preview your app or run it in a device.