Is slow cooking in an oven safe with long cooking times
In general, the issue is how long the food spends at the middle temperatures, warmer than refrigeration, and not as hot as truely safe cooking temperatures, the so called "danger zone" from 40-140 F, 4-60 C.

While an over simplification, pathogens do not grow well outside of the danger zone because it is so cold that they are inactive, or so hot that they are distressed or killed.

Slow cooking, done properly, is safe because the time spent moving through the danger zone to the safety of temperatures above about 140 F / 60 C is minimized, thus minimizing the opportunity for any pathogens to grow and create toxins; then, at temperatures above the threshold (if held for several minutes), most pathogens are killed.   (Do not take this to mean food is "sterilized" as spores can remain, that can become active later.)



Foods are not cooked to a single temperature to be considered safe.  Instead, there is a combination of time and temperature at which they are considered safe, assuming no other problems existed.   This is mere seconds at 165 F / 74 C, but perhaps half an hour (going by memory, so don't take this as gospel) at 135 F / 57 C.

Still, general audience food safety recommendations suggest temperatures of 160 F / 71 C because the 99.9% kill time at that temperature is only a few seconds--it doesn't require careful monitoring or timing, and so is a simple rule to follow.

275 F / 135 C (I see you edited the question) is definitely a temperature range for slow roasting that is safe when done properly.

135 F / 57 C is possible, but on the borderline, although it would require very precise control not usually available in an oven; this is more the realm of sous-vide cooking techniques.  It also requires longer periods held at temperature, and so is not a method for casual cooks.

100 F is not safe.  It is well into the temperatures at which food is being incubated so the pathogens have a cozy environment for growth.