Just two weeks after the last one I saw yet another zombie in diamond armor, and it dropped its chestplate, which had Protection II and Thorns I, which is only the third piece of diamond armor that I've gotten (I've also gotten a chestplate with Fire Protection I and Thorns I and leggings with Fire Protection I):
This is the third one that I've seen in two months, which is quite a lot considering that I've seen about 9 total over close to 29 months of daily playing, or more than 3 months between each one; prior to then the average was one every 4 1/2 months (one month is about 105 hours of playtime).
Also, I've explored around most of what appears to be the western boundary of the spawn continent within the current map (x = -3072 to -1024, z = 1024 to 3072); there is more land to the south but I don't plan to go any further (or as far as the current cave/mineshaft complex extends):
There may still be more land to the west of my current location (a swamp, not ocean), and there appears to be a desert to the south, near the bottom, but there probably isn't much more. I also still plan to fill in the gap to the north, for which I'll return to the previous base I made, to the north of the Mushroom Island to the west since it is closer than the current one, around which there are more caves extending eastwards which I had not explored at the time since I didn't know how much ocean there was (as I generally stay within a couple hundred blocks of land. I did not explore westwards from the land to the east since I didn't find any caves going further west, the extension to the north also has an unexplored mineshaft at the northern end).