Prior season. Starts at bronze 5. Current season Starts at iron 4.
Assuming Each Tier represents a similar range to previous seasons.
Assuming Iron 4 is the same as prior B5, The 4 Divisions would break to be.
Iron = B5-B2. Bronze b1-S3. Silver = S2-G4. G= G3-P5. Plat is = P4-p1. Diamond=d5-d2.
The entire ladder sinks basically. And tiers move slower then MMR so you see upticks in "lower" Tiers reaching into higher MMR games.
Because Your seeing players at G2-G1 being in last years Plat MMR.
Odds are... Riot might have tweaked the ranges of MMR they want each Tier to represent as well which can have impact especially if they offset the ladder to manipulate population placements.
You had 27 Titles last season. 5 for each Color then Masters+Challanger. so 5+5+5+5+5+1+1. You have 27 Titles this year. 4 for each and GM got added. So 4+4+4+4+4+4+1+1+1.
I don't think the ladder really got changed all that much but the titles just overlapped. You can cover the exact same ground but lower Elo players think they improved because you see more of them in silver-gold with much lower MMR range then last year. Then it starts to even out around Plat into Diamond thou the folks who previously had been in the bottom bits of D5 are likely to be Plat this season.