tldr:
low elo is iron to gold. scaling is being readjusted from a sliding scale of 54.5% to 52.5% win rate at 7-35+% ban rate moving to 54-52.5% win rate at 7-35+% ban rate.
High elo is being reframed from plat 4 - grandmaster to plat 4 - diamond 3. new sliding scale, formerly 54-52% at 7-35+% ban rate moving to 53.5%-52% win rate at 0-35% ban rate.
elite elo is being moved from purely challenger to being d2+. previously required a greater than 45% ban rate. Win rate is not a factor at this elo.
now requires greater than 50% ban rate on previous patch and current patch
or
54-52.5% win rate with 7-35+% ban rate. Win rate now is a factor
Pro play now requires 95% presence in a single patch for the auto nerf threshold to kick in from 90% for one patch. 85% over two patches remains unchanged. Now worlds patch and MSI patch have a 5% lower threshold for auto nerfs to kick in, from 0%. Autonerf thresholds for patches including the MSI/worlds patch is now 90% over one patch and 80% over two patches
In addition, riot is now monitoring the % total of champions that riot considers viable at all elos including pro play, and will nerf/buff accordingly beyond the thresholds. These will be long term changes, not short term knee jerk changes. Riot will not be creating solo queue or pro play problems to buff one of the numbers up, so the changes will be more long term. For instance, if riot is one champion under the low elo win rate threshold, they won't buff azir to create a pro play nightmare just to clear the threshold for solo queue.
The new thresholds for viability are
99% considered viable in all of SR games (normals and ranked)
95% considered viable in all of solo queue
80% of champions have a greater than 48.5% win rate in iron - diamond 3.
45% of champions have greater than a 7.5% pick/ban rate in diamond 2+
45% of champions have greater than a 5% pick/ban rate in pro play.
EDIT: GBW corrected me, it's not the patch of MSI/Worlds where the threshold is reduced, it's serveral patches before.