I hadn’t come across this term before looking at the crime writer John Dickson Carr’s bibliography - one of his titles is The Judas Window, 1936. Here’s OED:
3. A small aperture or lattice in a door, orig. the door of a prison cell, through which a person can look without being noticed from the other side; a spyhole, a peephole. See also Compounds 1b. Now somewhat arch.
1837 Ward’s Misc. 15 Nov. 727/2 Following the slow march of that whitish square that the Judas at my door cuts out upon the dark wall opposite to me.
And, under Compounds:
b. Designating a spyhole or similar opening in a cell door, etc. (see sense 3), esp. in Judas hole, Judas trap, Judas window, etc.
1858 Sat. Rev. 31 July 103/2 It is impossible any longer to ignore the existence of these ‘Judas-Holes’. Many English ladies will feel very uncomfortable henceforth.
1865 Daily Tel. 9 Nov. 6/6 The man..you may see through the Judas-hole when you make a round of the Model Prison with the visiting magistrates.
1866 G. A. Sala Trip to Barbary ix. 157 In some Moorish houses, in addition to the Judas trap in the door, there are spy-holes in the wall.
1874 Tinsley’s Mag. Aug. 219/2, I looked at him through my Judas window, as he took his accustomed place in the outer office.
1900 Methodist Mag. & Rev. Apr. 374/2 Through a ‘Judas’ slit in the door the guard could watch every motion of the prisoner.
1981 H. Engel Ransom Game (1982) xxvii 149 They..pushed me..up a narrow set of stairs. At the top was a door with a judas window in it, like in speakeasies in the movies.
1989 ‘D. Wait’ in K. Smith Inside Time xii. 81 All the old doors are also bolted… Almost gone now is the judas spy-hole with its pear-shaped flap.
2006 L. Welsh Bullet Trick (2007) 10 There was a brief pause, then a bustling beyond the door and a Judas hole slid back with a crack
Two points. Re that 1858 cite I wonder which ‘Judas-Holes’ were meant. Surely they weren’t built into private residences. Or, moving on to my second point, were they? From the wiki on Carr’s novel The Judas Window;
In the British prison system, a “Judas window” is in the door of a cell and enables the guards to observe prisoners without being seen themselves. But Sir Henry Merrivale points out another Judas window that is in every room, but that no one notices.
What does Sir Henry mean? No explanation in the wiki and I haven’t read the book.