This keyword has an effect on x86 32-bit platforms only. It allows the delivered image to run on old Pentium-compatible CPUs that do not support SSE2 instructions.
LispWorks 6.0 and later on x86 platforms uses instructions that are part of SSE2. All new CPUs have SSE2, but it may be required to run LispWorks runtimes (that is, delivered images) on old machines without SSE2. On these machines the SSE2 instructions are not implemented, and cause exceptions.
When :old-cpu-compatible is non-nil, deliver creates a runtime with a mechanism that checks for SSE2 on startup. If the runtime machine does not have SSE2, this mechanism then eliminates the SSE2 instructions. This mechanism allows the runtime to run on any Pentium-compatible CPU.
The cost associated with this mechanism is negligible, so normally there is no reason to change the default value of :old-cpu-compatible.
LispWorks Delivery User Guide - 15 Feb 2015