On Mon, 6 May 2019 13:14:49 -0400, Wade Garrett
wrote:

Performing routine annual maintenance on my Echo hand-held leaf blower,
I discovered that last spring, I mistakenly installed a spark plug one
heat range hotter than specified. The blower was used plenty with this
wrong plug in it- and the part of the plug inside the engine had a heavy
very light gray deposit on it.

Wonder if that did any permanent engine damage or shortened its life…

IDK. In a pinch I have used a higher heat range plug in a mower.
Never saw any problems that I could discern.