I remember web-safe colors and recording SMTP files onto cassettes. I've used Photoshop since it first came out. The first version I used was free -- it came with some other software.
The concept of safe/unsafe web colours is irrelevant nowadays. Back in the 90s computer graphics hardware was limited and couldn't produce more than 256 colours on-screen at 640x480 / 1024x768 resolutions, and with the advent of the web a colour palette of 216 entries was agreed on to prevent the GUI in Windows and MacOS from being affected by colour shifts. The GUI obviously required a set of fixed static colours to look correct.
The colours are still recognized by browsers, though, and many are actually named too!
At the time it was 'discovered' that only 22 of these 216 'web safe' could be reliably displayed without inconsistent remapping on 16-bit computer displays. These were called "the really safe palette".
I believe that means the color is Web Safe or falls within the 256 colors that a Gif or PNG-8 is limited to.
(that's supposed to be a Diamond symbol)
Since the Save for Web plugin dates from photoshop 5.5 (circa 1999 and based on ImageReady which is older than that) it could also mean the colors fall within the standard 216-color color table common to Windows and Mac OS 8 bit (256-color) palettes.
Actually according to the photoshop help, Web Safe is based on the 216 Color palette.
@scotwllm I can't duplicate it. What are the color settings of your original image? Do you get these plus signs with any image you save this way? Maybe it has to do with the number of colors in the palette, suggesting there are more?