I doubt diamond generation has ever changed. There's not much reason to tweak it unless there's a bug, especially when you consider that it's meant to be rare since its so powerful.
There have been at least four known changes to diamond generation, first in Beta 1.8, which reversed the direction veins "grow" in from up to down, which effectively lowered the maximum altitude from y=19 to 15 (the underlying range has always been 0-15), though this didn't change the density on the peak layers (currently 5-12; note that many say that 12 is best but it is better to be at 11 or lower; this is most likely because very old versions only showed your eye level, which is 1.62 higher than the feet level displayed by current versions). Even older versions, back to Alpha, appear to have the same ore generation as up to 1.7.10, as seen here (the names are obfuscated but this is clearly diamond based on the other similar loops above it, where the counts and vein sizes all correspond to the values in 1.6.4). Interestingly, some claim that it used to be much harder to find ores back then, though prior to Beta 1.6 there was a bug that caused ores to be much less concentrated than intended at negative coordinates, but positive coordinates had the intended amount).
Then, 1.13 made yet another change to ore generation, which initially lead to significantly less ore than before, which has been said to be fixed by the full release but somebody claimed (last comment) that 1.13.1 was still affected (the area they analyzed was only about 128 chunks but from comparing 100 chunk areas in a 1.6.4 world there shouldn't be more than about a +/-20% difference, even when comparing cave-dense regions (much denser than since 1.7) to areas without caves; they found only 2.26 diamond ore per chunk in 1.13.1 compared to 3.87 in 1.12.2. Unfortunately, the usual tools for analyzing worlds, such as MCEdit, are incompatible with newer versions so it is very difficult to confirm if this is correct).
That said, somebody did analyze the ores in a 1.16 world and found about 3.28 diamond ore per chunk, which if representative of the overall average (Large Biomes should make no difference, and never has previously, only affecting biomes) would indicate a decrease from 1.8-1.12 but not from older versions; I myself have never had any issues finding diamond in 1.6.4 (this branch mine yielded 91 diamond ore from about 5 km of tunnels, I only made it that large because I was looking for a much rarer mod ore and even without Fortune I'd need to find less than half as much diamond in vanilla), and the data given on this Wiki page, which claims that up to 1.7% of all mined blocks can be diamond ore, was based on an older version (the chart is from 2012).
The analysis given in this link is also interesting, it shows how many blocks you may have to mine before finding any diamond - about 7000 blocks in the worst-case, averaging 708 (note that 1.7% of blocks still means you may have to go much further before finding any diamond since they occur in veins; with an average of 4.3 ore per vein you'd have to mine an average of 254 blocks between veins):
Longest distance between diamond deposits on any path: 583 blocks
Mean/Average distance: 59.2 blocks
Distance you have to mine to cover % chance of deposit:
100%: 583 blocks (6996 blocks removed) (Valid just for my map, but higher maxes will be very very rare.)
95%: 168 blocks (2016 blocks removed)
90%: 131 blocks (1572 blocks removed)
75%: 81 blocks (972 blocks removed)
50%: 41 blocks (492 blocks removed)
Mining the 59 block average: 63.5% chance of diamonds (708 blocks removed)
Note that this was based on Beta 1.2 but as mentioned in the first link the density on the peak layers did not change until release 1.8, so this can be compared with the Wiki's data. They also used a spacing of 2 blocks between tunnels (one every 3rd block) as they mentioned that 5 tunnels covered a 15 block wide area, which according to the Wiki link is only about 1/3 as efficient (0.6% of blocks being diamond) as a spacing of at least 6, so the 254 blocks I calculated based on their figures isn't inconsistent with this showing 708; 1.7/0.6 * 254 = 720 (for comparison, my branch-mine had a spacing of 3, which averages about 0.9% of blocks being diamond, which closely matches what the Wiki's chart shows).
Also note that from 1.13 onwards you can find a lot more chests between shipwrecks and treasures from treasure maps so probably the average player finds with a few more diamonds than usual earlier in the game.