Whenever 32 GB memory modules became available since the typical ATX motherboard has four DIMM slots (4 x 32 GB = 128 GB). I don't keep track of such minutiae but it was probably 5+ years ago albeit it at astronomical prices.
Today, a 16 GB stick is a mundane commodity part. I have two such sticks in my mini-ITX motherboard. Not a big deal, I think the pair were $118.
I'm sure the 32 GB modules have been around considerably longer for the enterprise market.
Note that x86-64 architecture supports 52 bits for physical memory, thus a maximum of 4 pebibytes (PiB).
An Internet search for "32 GB memory module" will likely result in news articles that will narrow down the window of mainstream availability of this size memory module to the general public.
I'll leave it as an exercise to you to figure out which year it was.
My guess is that the iMac has been able to support 128 GB of RAM before 2019 but it wasn't a built-to-order option directly from Apple because they knew no one sane would pay Apple's inflated add-on memory price in the early days when it was available.
Even to this day, there are plenty of folks who buy Macs in a lesser configuration when it is possible to upgrade components. Today, this is limited to memory and storage for some Macs. For many Macs, there are no post-purchase upgrade options these days, everything is soldered to the motherboard.