Comment 2 for bug 1690582

I believe it is still needed. If you remove --not-show-in you'll get both keyboards showing at the same time. Out strategy so far was to disable Onboard in GNOME until it is explicitly requested. That request can come from installing and enabling gnome-shell-extension-onboard for example, which makes sure Onboard is launched.
There is an old bug report asking to resolve the clash between the two keyboards on Ubuntu. That behavior was one of the results.

I wish there was a simple setting that could switch between keyboards, but as far as I can tell GNOME Shell devs have made no attempt to provide one. The built-in keyboard has to be worked around with JavaScript "tricks". Our extension does that, but revives it for activities and password screens and then shows Onboard the rest of the time (something I'm hoping to change in the future).

org.gnome.desktop.a11y.applications screen-keyboard-enabled is handled by onboard-autostart.desktop too, you've probably seen it. Only --not-show-in prevents Onboard from being started.