Season 6 does 3 things to make teleport worse mid and top.

They are -

The Rift Herald - There's a new Jungle mob that spawns at four minutes in the baron pit, and respawns 5 minutes after being killed. It disappears at 19:45 to make room for baron, or 19:55 if it was in combat. It’s a tanky, high dps (much higher than dragon) mutated scuttle crab, with an eye on its back that opens on occasion. The eye can’t be hit by the person tanking, but someone else can hit it for 15% of its max hp in true damage - more than smite. This allows two people to take it quite quickly, while it is nearly impossible to take one on one. When killed, it gives everyone on the killers team 50 gold, and the killer itself a mini-baron buff. The buff grants 10% increased damage, but not true damage, 40 movespeed, a 40% buff to nearby minion attack speeds, and the 4 second recall of baron buff. While not as important as dragon, this objective gives top lane a strategic objective beyond just turrets, which in turn makes ignite or exhaust taken for lane dominance stronger relative to TP, and gives junglers and mid laners more incentive to gank or pressure either lane - top lane to take the herald (and top tower with the minion buff), or bot lane to distract the other team while top and the other take it. Overall, it’s a great strategic addition that will make top lane less of an island while lowering the dominance of TP, as TP is no longer necessary for top laners to influence the rest of the game pre-20 minutes in, as well as bringing reason for influence to come to them.

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Lowered XP and higher gold from jungle mobs for non-junglers - The jungle items were shifted from giving bonus gold on monsters kills to bonus experience, while monsters XP was shifted down and their gold shifted up to compensate. This is almost entirely due to the dominance of top lane either double jungling, were they tank two camps for the jungler, leaching xp and killing the small monsters, so that they both get level two with the jungler at full health, or the top lanner taking a camp for themselfs level one, then in either case recalling and teleporting top with extra potions and level 2. Once a again, this is a nerf to TP.

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_ And of course, a straight nerf to Teleport_ - TP no longer gets a 60 second lower CD for teleporting to towers. In all honesty, I would've kept that and instead just increased its CD by 30 seconds in both cases. The real issues are the jungling issues above, that TP is the best summoner mid asides from flash, TP being even better than flash top, and frequent teleports to lane wards bot lane to capitalize on overextension/aggression. This has really made bot lane much more passive than it used to. TP’s high usage is largely symptomatic of top lane being so strategically non-optimal for junglers and mid to roam to (In high tier play, a jungler or mid being spotted top, at least without TP, means the that the other team takes drag) TP is the only way top laners can see any action before 25 minutes, of ignite being use-impaired top and mid lane thanks to barrier, the meta, it’s low range, common shielding items, and current support picks (caugh caugh Tahm) countering the bursty playstyle that traditionally got ignite. This nerf however, is largely negated by the 15% cdr reduction on summoner spells early on in the defensive tree, and therefore quite accessible to any top or mid that wants it. Boots of lucidity now grant another 10% cdr on summoner spells, and are cheaper, while the distortion upgrade is much more viable thanks to a replacement of the near mandatory (or at least, the most noticeable and fun) homeguard upgrade with a buff enabled on everyone at the 20 minute mark, although this isn’t shipping with 5.22. Still, the CDR works with all summoners, so while it adds the lost power back to TP, it adds power to non-nerfed summoners as well.

This is from my section from my mega post dealing with all the systemic changes on PBE, The State of Marksmen, part 3 (What, Why, and How the Preseason Changes the Game.) (Megapost)