I started the project in 1995 while trying to program a 8051 development board. The first version was a very simple compiler, I released the executable (for free). There was an overwhelming response and many many feature requests. I decided to do a full featured compiler, it took me almost 3 years to do the first version, I think I released it in 1998, it was first hosted in the geocities site (now extinct), but regularly exceeded my monthly download quota within the first few days. It was one of the very early projects on Sourceforge, and still in the top 80 percentile (in terms of download).
2010-08-06 Sandeep Dutta wrote about SDCC original licensing, development platform, portability and retargetting
SDCC was GPLed from the very first release. The original target platform was an early version of linux called "slackware". All the portability was done by the author of the simulator msc51. The backend was globbed together. I did not think to much of retargetting in the first release. It was Michael Hope who did the splitting and added the z80 target.