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I know that it’s “too late” for the soy bean meal I just applied on my pop’s lawn but he had two bags left and I didn’t want them sitting there over the winter. That led to me wondering, at what soil temperatures does the break down of the SBM stop? Anyone have any idea?

For most biological beneficial activity that we are interested in 50 degrees F seems to be the magic number. Probably there is some minimal activity down to 32 degrees.

There is no magic number. Nature doesn't behave like that at all. Beginning at 55F, both the grass growth slows and microbial action slows, and the curve accelerates downward through the forties and into the thirties. They are pretty aligned, because nature works like that.

I should start taking my soil's temperature. I certainly have plenty of meat thermometers.
Science keeps finding microbes which thrive at extreme temperatures, so I would be surprised if the soil did not have some of those. Then the question becomes, are they of any benefit to the plants or the soil itself. I believe something beneficial happens in the winter time soils, at least in South Texas, because of the way the (St Augustine) grass awakens with such deep green color and vigorous growth. In my area of calcareous soil, it seems something happens to acidify the root zone and make last year's unavailable iron available. But if we get an early spring drenching rain, all that greening effect disappears and we could have a full season of chlorotic turf. It seems the rain can wash away whatever acidity we had leaving a high pH behind.

You can always check here, though a meat thermometer isn't much more work.

https://www.greencastonline.com/tools/soil-temperature

I just kinda figure surface temperatures (where the soy will be) are the average air temperatures give or take five degrees or so. Right now, that's right around 35.

Checking the above, they say 32.