I think there's merit in asking the question. I'm no landmine expert, but I'd expect that over the course of the war the quality of landmines developed (for example, the development of non-metal mines that were harder to detect), and the number of mines used was huge. They also played important parts in many battles (Kursk, for example, where the Soviets used at least hundreds of thousands of them). These mines didn't magically appear out of thin air, and their design wasn't an ancient recipe handed down from generation to generation that didn't require any research, and they absolutely had a strategic impact. They're not as sexy as a Tiger tank, but they were significant in blunting the Tigers during Germany's early advances at Kursk, for example.
Given that HoI4 has research for infantry equipment, having landmines in the tech tree or even in production isn't a crazy thing to suggest. Unless it was the case that landmines were used equally, to equal effect, by all combatants, in all theatres (so can be assumed to cancel each other out), which was clearly not the case, then there's value in modelling them in the game in some way, shape of form.
Don't get me wrong, I'm not suggesting the best approach is to individually produce and lay each type of mine separately

. Some ways it could get in there are:
- A simple tech modifier, like winter/summer equipment. I personally don't like this approach as it means once you've developed it you've got free mines for all. It'd be the equivalent of giving infantry weapons for free.
- A simple tech modifier (maybe with a few stages - basic mines, advanced mines and non-metal mines?) and some kind of choice to equip theatres with mines, which then increased the supply requirement. So in the Western Desert, you can have extensive minefields, but if you want them, you need to ask for them, and get the supplies to have them. You could even have different levels of mine provision in each theatre (my preference). Then if a unit in a theatre is dug in, the dug in bonus is modified by the level at which land mines have been supplied. A similar approach could be to use the commanding general as the toggle for level of mines available. If you went crazy, you could make this a division-specific modifier, but that'd become a lot of work very quickly.
- Actually producing mines as a separate piece of equipment, and designating whether there would be defensive minefields in place at specific defensive positions. While the most accurate, this'd also add in a tonne of micro, so don't think it's a great idea.
Of course, the game'll survive without modelling the impact of landmines - the franchise has so far, after all

. These are just thoughts for conversation.
Also worth noting that I'm pretty sure all major combatants used mines, and not just the UK, Germany and USSR. The US and Japanese definitely used them, and while my very rough research on the French is a bit sketchy, I'd be very surprised if they weren't involved in the Maginot Line at the very least. China is the only major I haven't been able to find anything on, but I haven't looked that hard.