I have observed that the second line will receive SMSs, but no iMessages. I have de-registered the second line from iMessage Deregister and Turn Off iMessage - Apple Support, and people text me like normal. iPhone, Android, regular flip phones, whatever. Since my second line is my work line, I don't care. I rarely send anything by text anyhow.
The one feature I miss from huawei and samsung devices, is the dual apps. I liked having 2 separate what'sApp. I some asian customers insisting on sending me what's app messages. Giving them my personal what's app is not an option for me. That duplicate app feature is really useful. That's the only think that I miss.
So in short, if deregistered, other iPhones should not try to send over iMessage, but should be able to send to you text messages via SMS.
Good luck.