Omicrons were removed from Galactic Challenges and put into the era store instead. Initially, there were slots to buy 4 a week (more or less mirroring what he had before from Challenges) but shortly after they put in another slot to give us the opportunity to buy 5 a week.
So they've actually been made more available than they were.
Since Galactic Challenges have been removed, I'm noticing slower progress through the era reward track - if this led to a week where I didn't have enough currency to buy all 5 omicrons in the era shipments, that would be a concern. This hasn't happened to me yet.