I'm frankly really disappointed in this decision.
Since the creation of League, new players have not needed to climb through the depths of bronze. If you did well enough in placements, you could start in medium-high silver or even gold.
I myself went 8-2 in my first ever placements back in season 5 (after playing almost a year in normal draft, waiting to have enough for solid Rune pages, etc...) And got placed in Silver 1. I subsequently felt that I had some games with bad teams and fell to silver 2 0lp, then climbed to gold 5 by the end of the season.
The only time I've ever been bronze was after the dynamic queue fiasco, where I placed bronze 1 in the new dynamic queue for going 5-5 in placements after they brought back solo queue (where I placed gold). And it took me more games to climb from bronze 1 to silver 5 than it took me to go from silver 5 to gold 5 afterwards. Playing in bronze was a nightmare - too many games were simply Unwinnable.
Now, all new players will go through that - and I don't believe any warning was given. I'm picturing if I had started in late season 7 and did the same strategy of playing a lot and being roughly gold quality before entering ranked to have heard this news. My climb was just made about 200-250 games longer due to my not starting ranked a few weeks ago instead.
Honestly, that's a terrible policy. A standard was there for a long time. If you want to change it, that's your choice. That sets a standard of "enter ranked asap" because you're going to start at the bottom anyways. No sense actually preparing and entering ranked knowing what you're doing.
If you're making that kind of change, it really should have been well advertised. Existing non-ranked players should have had the option to be treated like everyone before them was in their first ever placements.