flying below S speed is only mentioned on FCTM for the approach scenario.
No it doesn't, but it does say when you get to F speed, select flaps 1. It doesn't say "ïf you are going to accelerate above S speed then select flaps 1". So if your S speed is 200kts and your F speed is 150kts, but you have a speed restriction of 180kts on the SID, if you follow what the FCOM says, you will select flaps 1 at 150kts and accelerate to 180kts, but will be still be below S speed.