In a recent interview with PC Gamer, game director Ion Hazzikostas has revealed that Blizzard is auditing the Mythic+ ladder. Their goal is to remove paid boosted players from the list of eligible characters for 
Umbral Ashes and 
Umbral Champion: Midnight Season 1.
The reason that the Season 1 top-1% rewards haven’t gone out yet is that Blizzard is auditing the Mythic+ ladder before distribution. Ion Hazzikostas revealed that the team will publish the methodology and the results of the audit at a future date.
Note that the interview isn’t specific about what kind of ‘paid boosting’ is being deemed ineligible for the Top 1%. It doesn’t clarify if the boosts in question are only those done via RMT or boosting communities, or if it applies to any paid boost in general, even those paid with gold. We will likely need to wait until the official blue post is out to learn more about it.
PC Gamer interviewed Senior Game Director Ion Hazzikostas, Associate Game Director Paul Kubit, and Game Designer Andrew De Souza.
Spec-specific Cutoffs On The Table
In the interview, Blizzard highlights that it is happy with percentage-based rewards in Mythic+; however, they seem to be experimenting with how best to deliver it. One of the options highlighted is the addition of spec-specific cutoffs for future seasons. This, however, is a future plan, not happening for Midnight Season 2, as there are logistics that the team needs to discuss, but they want players to know they are investigating it.
Both Ion Hazzikostas and Paul Kubit call the 1% rewards popular during the interview. Kubit even highlights how percentage-based rewards keep players engaged, keeping top contenders fighting for those spots until the last minute. You can contrast this against flat marks (like reaching 3400 score) leading to players “finishing” a season.
Season 1 in Review: Record Participation
Based on data aggregated by Raider.IO, Midnight Season 1 saw 5.2 million characters participating in Mythic+, a large increase from 2.5 million in The War Within Season 1. However, while the total number of characters was high, end-of-season retention saw a large drop, with only 12.3% still running keys, versus 16.8% in TWW Season 1.
Flavor of the Month Problems
Not everything is sunshine and rainbows. Although participation was way higher in Season 1, homogenization was a large problem. At +20 and above, you basically saw only the same seven specs being played: Guardian Druid, Unholy DK, Devourer DH, Mistweaver Monk, Augmentation, Resto Shaman, and Ret Paladin.
To better see the homogenization problem, you can look at Guardian Druid versus Blood DK. Guardian recorded 518,467 runs at +20 and above, while Blood DK reached only 7,635; this is a 67x gap between the specs. Kubit highlights in the interview that “[players] shouldn’t feel forced into any particular meta spec”, however, this is not what happened in Season 1.
Time is a Flat Circle
There is a parallel that can be drawn from this situation in WoW itself. PvP started with a similar system all the way back in TBC, with Arena rewards being tied to a percentage rating. As seasons went by, the system evolved, with rewards slowly migrating to flat values instead of percentages, starting around Mists of Pandaria.
The change made during Mists was contextualized back then as a form to combat boosting, which had grown rampant. The first change was the flat rating for rewards and then the requirement of 50 wins for being eligible for them. There is an argument that PvP ratings fluctuate up and down, but, overall, the situation isn’t much different.
Resilience Won’t Fix It
Another aspect that is important to consider is Resilient Keys. A controversial topic in the community, but, like them or not, Resil Keys play an important aspect in boosting, as they allow boosters as many retries as they can stomach without much consequence.
A Change of Policy?
Note, however, that these changes, should Blizzard go ahead with them, are likely a policy change. Blizzard’s stance on boosting has been nebulous in the past, but so far they have never gone so far as to remove rewards from “legal*” boosts. That is, boosts done with gold and not via RMT. Removing eligibility for boosted characters regardless of how the boost was done seems to be a policy change.
*Note that legal here means boosts bought with gold from individuals/guilds without any practice that breaks the EULA (like Piloting or account-sharing).
Note, however, that Blizzard never officially publishes the reasoning about DQ in PvP or Mythic+, so this is all community data. It could be a policy at Blizzard for a long time, just something that players have never noticed.