Which macOS version was the first to require an Intel processor, dropping native PowerPC support?

The first macOS version to require an Intel processor and drop native PowerPC support was OS X Snow Leopard, version 10.6.

Released on August 28, 2009, Snow Leopard followed Leopard, which was the last major Mac OS X release that could boot natively on both PowerPC and Intel Macs. By then, Apple’s transition to Intel hardware was effectively complete: the company had introduced its first Intel Macs in January 2006 and had moved the Mac Pro and Xserve lines to Intel later that year.

Snow Leopard was designed chiefly as a refinement release, emphasizing performance, efficiency, and a smaller memory footprint rather than a long list of headline features. Its Intel-only requirement reflected Apple’s effort to focus development on newer hardware and technologies such as 64-bit computing and OpenCL.

The subtle point is that Snow Leopard still included Rosetta, Apple’s PowerPC translation technology. That allowed many older PowerPC applications to run on Intel Macs, even though PowerPC hardware itself could no longer run Snow Leopard. Rosetta disappeared in OS X Lion, making Snow Leopard the last Mac OS X release capable of running PowerPC-only software.