Would it be possible to add an option where below a (configurable) y level crops from (configurable) seasons grew without greenhouse glass above, instead using the warmth of the earth?

I'm running 1.12.2 SereneSeasons and Forestry's managed multifarm appears to grow vanilla crops regardless of season.

Well that doesn't seem right. For me it was following seasons fine, but it has been a while since I looked at it.

Is this option not going to be made available to 1.12.2 users?

No. We don't support older versions. It's enough work keeping one version updated

I figured as much, and I don't blame you. I'll see this in a couple years then when I actually get fully moved over to a newer Minecraft version (big mods move at a glacial pace).