Shadowbanning like that could definitely exist without any player knowing about it; how would you prove it without the counter-argument that "you just had bad luck"? Before a large enough sample size was gathered, the player would likely give up out of frustration.
Mod Mat K may have hinted for the possibility that shadowbanning similar to this might exist, and confirmed that they did at least consider it before:
I think most of us just do get hit by simple crappy RNG from time to time.
I'm not convinced that some vengeful JMod has the wherewithall to tinker with individual accounts though, something like that would surely take a lot of programming mucking about and would Jagex (A) think it was worth it just to punish someone for being naughty and risk them quitting out of irritation and (B) think it was actually an ethical thing to do.
I had an appalling relationship with a JMod for some years prior to him 'moving on' and I don't doubt he would have hit my RNG out of spite if he had been able. But I never noticed any variation in my RNG.