Following the discontinuation of its iMac Pro, Apple had announced that it soon discontinue Space Gray Magic accessories and starting this week, the company has indeed stopped selling the Magic Keyboard, Magic Mouse 2, and Magic Trackpad 2 in Space Gray colour (via Appleosophy).
When Apple debuted the iMac Pro back in 2017, it did not make any of the Space Gray accessories available as separate purchases. However, the company started offering them as standalone accessories a couple of months later.
Since Apple has now discontinued the Space Gray iMac Pro, it makes little sense for the company to continue offering matching accessories.
Apple, however, continues to sell silver/black accessories to complement the Mac Pro, and has recently also started offering colour-matched Magic accessories to complement the new M1-based iMac.
