Where does Futaba say that at ?
I suggest everyone read the manual until you understand what it is saying. So many think they can wing it then are upset when they smoke a analogue servo. Not sure why anyone would still have one of those but in any case.
The radio is trying to tell you it's time for digital. Lol.
The R324SBS is perfectly fine running analogue if you set it to "normal mode"
The problem is that people do not know to cycle the power switch on the transmitter after making the change. If you don't do that it doesn't make the change and if you plug in a analogue servo thinking it is safe it isn't and majic smoke again.
Important,
Please remember that this setting stays the way you turn the transmitter on so even if you change from a model number that was in high speed mode (DIGITAL) and you changed to a model number programmed to normal(ANAOGUE) mode
you can not turn on that model YET without first power cycling the transmitter or it will still be in digital and fry your servos.
https://futabausa.com/wp-content/uploads/2018/09/R324SBS.pdf
https://futabausa.com/wp-content/uploads/2018/09/3PV.pdf