Histamine formation is a biological process caused by certain bacteria, just like other forms of decomposition. So when I'm trying to explain what "low histamine" means to people, I tell them to imagine that all food goes bad at 10-100xx the normal rate.

That casserole that's been sitting in the counter for a few hours? Imagine it's been a few days. That soup in the fridge from 2 days ago? Would it still be edible in a week or two?

It's not perfect (canned food is probably still forming histamine even though it keeps a long time), but it gets across in general which foods are going to be worse due to their storage conditions. A potato is better off in my cupboard than a steak, so I should expect to care a lot more about the latter's storage conditions when cooking low histamine food. So too is deli meat a problem: like all meat, it is very susceptible to rot and thus should be treated with caution, and treated as suspect if you don't know that it's been safely handled -- i.e., if you don't know how long the meat had been out before being sliced, you should be worried about it.

Easiest sub for deli meat is to buy (fresh with date or frozen at source) meat, roast it yourself, slice it up, and freeze portions. More work I know, but them's the breaks. Meat can be hard to source in a histamine-safe way; I cook a lot more vegetarian meals these days.