Almost all of our aircrafts doesn't provide "minimum speed reversion" after G/S capture. Only some of the oldest ones provide that feature. So it seems Boing indeed removed "the feature" (or customer option). And worse you can't see it, as the A/T can still be deselected into ARM, so if you don't look in the manual, you won't know if your aircraft for the day has it or not.
Besides from that, the reason it's not recommended is most likely because it can be confusing when the throttle is armed and when not. If you do a VNAV approach, you HAVE to disengage the auto throttle completely.
So in a stressed situation, when flying a VNAV approach, a confused pilot might press TOGA without manually advancing thrust, and nothing happens!!
Better to keep uniform SOPs, so the pilot will always react the same, regardless of which type of approach is being flow. If you can't do a go-around with manual thrust, retraining is needed (badly)!
For the low speed scenario, Boeing invented the "airspeed low" GPWS call out. If that doesn't wake you up and cause you to advance thrust, you shouldn't be sitting in that seat either.
(...and I DO fly with ARM mode as it's SOP in my company).