Anything about game-mechanics which is on-topic is by definition game-design. I could not think of a counterexample, so I've proposed a synonym.
Supporting evidence: gameplay is already synonymous with game-design and surely game mechanics are a subset of gameplay.
The example of "How do I implement X mechanic? I love it in Game Y, and would love to expand upon the idea but I can't find anyone who explains how it is implemented programmatically" does not really touch upon game-design. It isn't asking why one would use the mechanic, the effect it has on the player or on other systems, or the meaning, or balance, or any other such idea. It isn't about game design, but is without doubt on the topic of game mechanics and game development.
There is some grey area between game-mechanics and game design, but they touch on slightly different areas of game dev. Design is more narrative and story driven, while mechanics focus on the type of actions the player may take and how the player uses actions to move the narrative. Design also involves guiding the player's decisions through game quest.(I.e. a red carpet travels to one room, but not an other, or the player hears rumors about a beast in the sewers, so they go there.) Both areas work in tandem to create the player's experience.
