DEF is used on >75 HP units as the regulations for emissions tightens at >75 HP, and as far as I can tell, nobody has found out how to meet the regulations at >75 HP without the catalytic reduction (SCR) that DEF is used to accomplish. I am not aware of any unit 75 HP and less using DEF as it is not needed to meet the 75 HP and below regs.
The 26-75 HP units all have an oxidation catalyst (DOC) to meet the various non-particulate emissions regs (hydrocarbons, nitrogen oxides, etc.). Some use EGR and high EGTs to keep the catalyst face hot enough to burn off soot so they don't need a particulate filter (DPF) to meet particulate emissions regulations. This works unless the engine isn't run long enough or hard enough to get EGTs high enough to burn off the soot, in which case you get catalyst face plugging and the DOC has to be removed and cleaned, or replaced if it can't be cleaned. EGR has its own issues too, it cruds up the intake, reduces the amount of oxygen in the charge air (which decreases power), and either increases the temperature of the charge air (which decreases power) or requires an EGR cooler to cool the recirculated exhaust. EGR coolers can plug up or fail and cause head gasket failures, just ask anybody who owned a truck with the Navistar VT365 engine, aka the "6.oh-no," about that one.
If they do not want to use EGR or use EGR but do not want to keep EGTs persistently high, they use a DPF in front of the catalyst to trap the soot and then use a temporary increase in EGTs is used to burn off the trapped soot (regeneration.) The no EGR setup gets rid of the issues with EGR and EGR coolers, and having an engine setting that can clean out the DPF eliminates the face plugging issue, but requires some user intervention in relation to the DPF regeneration. Otherwise, the DPF will get plugged up enough that the regeneration cycle won't burn off the soot and it will require removal and cleaning, or replacement if it can't be cleaned.
There is no free lunch when dealing with the feds, unfortunately, only different kinds of bad.