Then explain to me why I Never received the warning that I was in danger of demotion? I'm currently around 900 MMR after losing over 300 from my demotion to b4 from placement matches where I had a positive win ratio with 4 of my 6 wins coming in the first 5 games I played.
As I said you're original comment was and still is false. Without the 300+ MMR loss from placement demotion I would be over 1200 mmr putting me in at minimum b1 and no where near 0 LP in b4.
Because probably it wasn't near the demotion. Having Bronze IV MMR while being in Silver V does not lead to demotion. Only having Bronze V MMR does and then you'll get your warning. And you'll still be demoted to Bronze 1, not Bronze V (even though your MMR would be a Bronze V). So you see, this system is pretty confusing and super forgiving.
First you have to realize that after you finish placement matches you're not being "demoted". You can't be demoted from something you don't have. When season 4 ended, your rank has been archived. You no longer have it, that's it. Then system takes your MMR at the end of the season adds 1250 (or 1200, not sure about that) and divides it by 2. That's where you'll start next season. After that, where you'll be placed is solely decided by your MMR after the 10 placement matches.
And there is a phenomenon caused by the current League system being too forgiving to players. I know it can be hard to accept, but this system is designed to lie to people about their MMR to fight "ladder anxiety". Let's say you just got 900 MMR after placements and it got you placed in Bronze 4. You can spend months winning/losing climbing/falling and you can get to Silver 5 and you will still have that 900 MMR. Because of how forgiving League system is to your LP compared to MMR. LP is arbitrary points, without clamping you can trade 1 win for 2 losses at 0 LP endlessly and then climb a division while being 50 MMR down. MMR itself is strict, unforgiving and clear. But Riot thinks that it adds too much pressure. So only when the placement matches come, players can finally see where they would trully belong if they were distributed into Leagues ONLY according to their MMR. But soon they'll start playing regular games and the League system will allow them to climb without raising MMR. Lots and lots of people who float in the same ranked tier back and forth most of the times have their MMR much lower than their League is supposed to have.