When trying to learn more about it I found a Snopes article at https://www.snopes.com/fact-check/the-price-they-paid/ discussing a letter going around titled "The Price They Paid" making the claim "Five [of the 56] signers were captured by the British as traitors and tortured before they died."
Snopes comments that while five of the signers were indeed captured, four appear to have been treated as normal prisoners of war after being captured in battle ("they were not tortured, nor is there evidence that they were treated more harshly than other wartime prisoners who were not also signatories to the Declaration") and the fifth - singled out for signing - was sent to prison.
Wikipedia comments that the fifth, Richard Stockton, was "intentionally starved and subjected to freezing cold weather. After nearly five weeks of brutal treatment, Stockton was released on parole with his health ruined." quoting a 1823 biography.
But was that torture? Reading the biography, at https://archive.org/details/biographyofsign03sand/page/100/m... , the issue appears to be that rich Mr. Stockton, used to living the high life, has been placed in the commoner jail rather than the comfy jail for rich people:
> On his removal to New York, he was ignominiously consigned to the common prison, and without the least regard for his rank, age, and delicate health, for some time treated with unusual severity.
As Congress described it, "that he hath been ignominiously thrown into a common gaol, and there detained".
And the common jail had horrible conditions. Quoting https://www.americanwars.org/american-prisoners-revolution/p... :
> "The New Jail was made a Provost Prison, and here officers and men of note were confined. At one time they were so crowded into this building, that when they lay down upon the floor to sleep all in the row were obliged to turn over at the same time at the call, 'Turn over! Left! Right!'
> "The sufferings of these brave men were largely due to the criminal indifference of Loring, Sproat, Lennox, and other Commissaries of the prisoners.
> "Many of the captives were hanged in the gloom of night without trial and without a semblance of justice.
It doesn't seem like any of the founders were tortured during the war.