https://nexus.leagueoflegends.com/en-us/2018/04/dev-updates-to-ranked-for-2019/
Next year, you’ll get a provisional rank after your first game that’s only visible to you; this is the lowest you can place at the end of your placement games. Win or lose, your provisional rank will be lower than you ended the previous season, but you’ll know what to expect upfront. Over your remaining nine placement games, your LP gains will be accelerated, you’ll skip promotion series, and you’ll be shielded from losing LP since your true rank should be higher than your provisional rank. After you complete all ten games, you’ll unlock your actual rank and other players will be able to see it on your profile.
As far as starting at the absolute bottom of the bracket, riot made the decision to place anyone who was unranked in season 8 at iron 4, 0lp. Your MMR doesn't reflect that though, and you will still be playing with people at your skill level. If I'm not mistaken, they are still making tweaks to your first 10 provisional matches and rank, and I believe(?) that placements and lp gain are bugged currently.
Several diamond+ players got placed in iron 4 on alt accounts that they didn't play in season 8, but are still playing against diamond players despite their rank being iron. CoreJJ went 10-0 in placements and got iron 1, 65 lp, which I certainly hope riot doesn't think is acceptable.