Some rioter tweeted that most people should place about a tier below where they ended the season. No idea WHY they decided to do that, but it's apparently not a bug or anything.
this is normal and intended, and has a lot to do with how the elo system works and why Riot chooses to do resets each year.
2 important things:
1. Elo is not truly reflective of your ability, it's your Elo compared to other people's.
2. net Elo gained is slightly more than net Elo lost in LoL. this leads to Elo inflation.
look at this graph from early Season 3:
3 years ago,
48% of NA was in Bronze.
now,
look at the same distribution from the end of last season.
Less than 20% of NA is in Bronze. (bonus: the Ranked Teams ladders look almost exactly like 2013, because they have far fewer data points and thus far less inflation)
now, the reason isn't "the average skill of players has increased over the last three years!" (which is probably true, but ultimately irrelevant). the reason is because of elo inflation!
1200 Elo is "supposed" to be average. 1200 historically has reflected Silver V MMR, with each division being approximately 50-100 MMR over the last (Gold is supposed to be ~1500, Plat ~1800, Diamond ~2100, Master/Challenger is literally off the charts). if you look at that end of season 5 distribution to find the median NA player, you'd see they're about Silver 3/2 borderline. This means their Elo is ~1400, and that the average league Elo is 1400.
each season's reset is supposed to do two things: it pulls everyone towards the average elo, and it pulls the average elo closer to where it's supposed to be (ie lower). in general, someone with 1400 elo last season (Silver 3/2) should be at 1200 this season (Silver 5) despite still being an average player. a Plat player with 1800 Elo would get pulled down to ~1500 (only 100 elo over the "average player" last season, but 300 over the "average player" after a reset) and be about Gold 5. this is assuming they go 5-5 in placements, which generally count 5x more than "regular" games and can shift your elo 50+ points (so a Plat V player going 6-4 could easily end up in Gold 3, but is unlikely to be back in Plat).
the actual numbers are definitely not concrete and should not be relied upon for accuracy, but in general if you had normal MMR gains towards the end of last season you should expect a drop of 3+ divisions, more the higher up you are. if you were in division V of any tier you should expect to drop a full tier, unless you had just recently earned that new tier.
while doing your placements, just remember that elo gets inflated during the season. you might be in Plat 5 last year but Silver 1 now, but you'll be playing against the same people you played against in Plat 5 more or less. in 6 months, those people will probably all be in gold/plat again even with a 50% win rate.
honestly they could fix all this by making elo visible, but m-muh ranked anxiety