Odds are, it never made it past playtesting or the Vorpal Editor
The tweet in question came out (07/14) before any of the official playbooks (PHB was 08/14 and the DMG was 12/14). So it would seem likely that WotC had planned for such a thing, but it got scrubbed before launch for unknown reasons. Perhaps a tweet to JC reminding him would answer that question.
If you want ultimate control over the speed of leveling then use Milestones (DMG, p261). In that way, you set the pace. This also gives the feeling of "chapters" to a character's life.
Having rid all the basements in the town from rats, the party is thrown a celebration. Congrats, you are all now level 2.
This also helps if you have players missing sessions. If using straight XP, then you'll have people fall behind. With milestones, they all stay at the same level. I personally prefer Milestones in my campaigns.
Adventure League play uses a Milestone variant based on hours of play. The system changes from season to season but the basics are that for ever four hours of play you gain a level until you reach level 5. After that it's eight hours of play. Players can opt not to level up if they want to stay in a lower tier, for instance if they are waiting for a friend to catch up. 1
But I want XP!
The only way to slow down or speed up leveling and still keep XP numbers is to either change the spacing between levels (Eww, lots of math), or add a "multiplier" to XP given (Ehh, simpler math).
Since changing the rate at which characters level is so 1st-Edition 2, I would recommend using multipliers. It's a similar concept to how some video games do it.
So let's use a Gnoll as an example: Challenge 1/2 (100 XP)
When characters are lower level, Gnolls can be a threat. But at higher levels, they become fodder. So depending on how close the APL is to the CR of the monsters, they get full value, or a multiplier of (x1). After a while, the party levels up and the multiplier becomes (x.75) or worth only 75xp per kill. Then (x.5) and they are worth 50xp, and so on. There is a diminishing return for fighting Gnolls. So the party still gets XP, but at a much slower rate.
So to slow the parties leveling, apply a fractional multiplier to all their XP. To speed it up, make it a value between 1 and 2.
I'm no good at multiplying!
Then make all monster XP half, but add on bonus XP for situations.
- 5 Gnolls = 500 xp, halved = 250xp
- It was an ambush so the party gets an extra 200xp
Basically, it defaults to slow, but you can add bonuses to make up the difference and speed things up when needed.
Disclosure: I've only ever used standard XP and Milestone leveling because I'm okay with those systems. So I cannot attest to the other methods other than many RPG video games use a multiplier system and it seems to work for them.
1 This is just the basics. I'm not trying to explain the whole thing so please hold off on comments/downvotes explaining why my explanation is wrong.
2 Look it up. In 1st-Edition, each class needed different XP caps per level. So a Fighter leveled faster than a Magic-User even if they had the same number of XP.