Hey there! I'm the product manager who runs the Boost Nomination Pilot. The percentage of nominations approved for Boost was at 63% for June -- actually up from the program average of 60% approval level. So, I wouldn't say there are more declines than there used to be.
That said, rejection is certainly a difficult part of this program. When I'm training nominators, I try to remind them that rejection is an inherent part of writing, not just on Medium, but everywhere. Inquiries to magazine editors go unanswered. Pitches to literary agents sit ignored in slush piles. Even successful published authors will have their books declined by numerous book editors before finding a home for a new book.
As creatives, part of the process of expressing ourselves is navigating the difficulty of rejection. That's why I love that Medium is an open platform -- you may not get Boosted, but you can always share your story.