The fact you could make giant profits by buying from ge and selling to gen store due to jagex over-valuing item was obviously not intentional and therefore a bug.
People abused said bug and got banned for it.
There is no risk of being banned for playing new content - the only risk is if you are dumb enough to go out of your way to exploit something that is very obviously too good to be true.
Making big gains in short time is not something that can indicate ban.
1. Recently jagex relased new skill. Some people had 200m xp after couple of days, XP rates were abnormally high. No one was banned.
2. "too good to be true" is very subjective. For players with 10m gp: 5m gp/s is high For marchanter with phats and 2,1b it will not be as high.
3. Power creep is how jagex handle game. Every content is better than previous.
Just because you have previously been able to sell to stores doesn't magically negate the fact it was very obvious that it was not intended for players to be able to make giant cash stacks for doing nothing pretty much.
If something was 100% legit in past then players should be safe to asume it is safe with new content. Otherwise every new content could give you ban.