Depends

Are you doing a full 13 system or 13R??

You can fill it with non combustible insulation

From NFPA #13 2019 Edition

9.2.1* Concealed Spaces Not Requiring Sprinkler Protection.

A.9.2.1 Paragraphs 9.2.1.3, 9.2.1.4, and 9.2.1.5 do not require sprinkler protection because it is not physically practical to install sprinklers in the types of concealed spaces discussed in these three exceptions. To reduce the possibility of uncontrolled fire spread, consideration should be given in these unsprinklered concealed space situations to using 9.2.1.7, 9.2.1.11, and 9.2.1.13. Omitting sprinklers from combustible concealed spaces will require further evaluation of the sprinkler system design area in accordance with 19.3.3.1.4.

9.2.1.1* Concealed spaces of noncombustible and limited-combustible construction with minimal combustible loading having no access shall not require sprinkler protection.

We've all seen these spaces and if anything meets the minimal combustible construction it would be here the empty soffit space above the cabinets.

What do we have in there that would really burn and what would start a fire?

Limited-combustible has a specific definition and not sure a soffit consisting of 2x framing (assuming that's what it is) would qualify. If it's open to the overhead sprinklers you need not worry but if it's truly completely concealed I would put a sprinkler in it or fill w/ noncombustible insulation.

3.3.16* Limited-Combustible (Material). Refers to a building construction material not complying with the definition of
noncombustible material that, in the form in which it is used, has a potential heat value not exceeding 3500 Btu/lb (8141 kJ/kg), where
tested in accordance with NFPA 259, and includes either of the following: (1) materials having a structural base of noncombustible
material, with a surfacing not exceeding a thickness of ⅛ in. (3.2 mm) that has a flame spread index not greater than 50; or (2)
materials, in the form and thickness used, having neither a flame spread index greater than 25 nor evidence of continued progressive
combustion, and of such composition that surfaces that would be exposed by cutting through the material on any plane would have
neither a flame spread index greater than 25 nor evidence of continued progressive combustion, when tested in accordance with ASTM E
84, Standard Test Method of Surface Burning Characteristics of Building Materials, or ANSI/UL 723, Standard Test Method of Surface
Burning Characteristics of Building Materials.
The definition of the term limited-combustible material is from NFPA 220, Standard on Types of Building Construction, and is used in
portions of Chapter 8, specifically Section 8.15, where describing those areas in which sprinklers are not required. The presence of
noncombustible or limited-combustible wall materials is necessary to meet many of the exceptions that allow for the omission of
sprinklers in certain areas.