I think Mencken's choice of the word to express the passage of a few decades in an unhappy one. To me it means centuries not decades.

"The English critics, in fact, had overdone the thing, and though their clatter was to keep up for many years more, they no longer spread their old terror or had as much influence as of yore. "

Source: The Collins English Dictionary © 2000 HarperCollins Publishers:

yore [jɔː]
noun
1 time long past (now only in the phrase of yore)
adverb(ial)
2 (obsolete)
in the past; long ago
[ETYMOLOGY: Old English geara, genitive plural of gear year; see hour]