It's not reportable toxic, but it sends a negative message throughout the team. Basically you are saying "my ADC sucks, don't gank this piece of trash" in a roundabout way...or at least that's the way it's gonna sound to him. I get your logic, but honestly just missing one follow-up doesn't mean that he's bad. He may just have not noticed or understood immediately or heck even been in a position to follow-up. My Diamond 3 ADC main friend has missed a few of my opportunities several times just because even though we play together a lot, he's not perfect. ADCs aren't mind readers so communicate that you're going in. I'd leave the decision to gank to the jungler at that point providing warding information as needed. He's reading the map and bot may just be the best place to gank. If ADC doesn't follow, it's still a 2v2 and maybe a missed gank is what the ADC needs to wake him up.
I'd 100% not say anything about not ganking in chat unless it's something like "they are warded up well and playing safe plus have ton of disengage -- camping mid is a better idea, we'll hold them off"