It's curious that the Synths were all dismantled after the attack on Mars. The holograms, like the one at the Archives, sure; they don't appear sentient. But the Synths seemed to be modeled after Data... Were they, say, half-sentient? Could the government dismantle thousands of sentient or partly-sentient artificial lifeforms?
Lets take Maddox's sentience tests
Intelligence -- No real evidence they could adapt to brand new situations, certainly any more than a starship computer. They were programmed to cope with certain situations, but so's siri.
Self awareness -- Again no real evidence. We didn't have much evidence either way, but there seemed to be far more self awareness from some holodeck characters
Consciousness -- How can you prove anyone is conscious?
Lets look at Crusher's musings with the exocomps on whether something is alive (a lower requirement for sentience)
"Well, the broadest scientific definition might be that life is what enables plants and animals to consume food, derive energy from it, grow, adapt themselves to their surroundings and reproduce. "
For some reason it doesn't apply to Fire or Crystals. Synths presumably take in fuel, but do they grow or adapt themselves? Data certainly does, but we had 150 episodes to see that, not 150 seconds.
If we take a level of sentience which includes
Data, Lore, Julianna, The Doctor (Voyager), Exocomps, the wierd things in Phantasms, and I think Moriarty
But doesn't include
The Enterprise D computer, the residents of Fair Haven, Leonardo, the characters in Dixon Hill
I don't see evidence that the Synths are even approaching the level of Leonardo or Michael Sullivan, let alone Moriarty