I have been always care about the compatibility. Fcitx 5 will be able to type with either fcitx 4’s im module, or fcitx 5’s im module.
Fcitx 5’s im module will announce itself to accept both “fcitx5” and “fcitx”. Even if you did it wrong or forget to upgrade, application will still be able to pick up one of the fcitx im module. Even if it is the wrong one, the latest fcitx 4 im module is compatible with fcitx 5 server. There is no more excuse if you configure it wrong.
Even today, I just commit the code to even support ibus im module. So, when you start fcitx 5, it will automatically replace any existing ibus and initiate a shim dbus layer to provide ibus input context interface. So even your system is wrongly configured or forced by your desktop to use ibus im module, you should still be able to use fcitx anyway, as long as you didn’t forget to start it