As you note, "pre-canon" customarily means "before the first episode" (or issue, volume, etc.), regardless of flashbacks or time-travel, and seems usually to focus on the time somewhat recently before the present-day canon, not flashbacks, history, or time-travel. This may be because people generally consider the unfolding present-day of the story to be the core, most important, part of the series.
If one chooses to go with a definition of "pre-canon" that means only Lacroix and Divia stories can be pre-canon in Forever Knight, that same definition would mean that only Methos stories (in 3000 BC or earlier, early Bronze Age) can be pre-canon in Highlander, and no stories at all can be pre-canon in Doctor Who or Hitchhiker's Guide. It would mean that WandaVision pre-canon allows only Agatha's youth in the 1600s. Granted that flashbacks are not in most series, I've personally never yet seen someone define pre-canon in this exclusive, flashback-focused way...?
Perhaps what one might want instead of restricting the definition of pre-canon to this extent is to invent a second, new category with a new, different name for this narrower, stricter slice of pre-canon content?
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If one chooses to go with a definition of "pre-canon" that means only Lacroix and Divia stories can be pre-canon in Forever Knight, that same definition would mean that only Methos stories (in 3000 BC or earlier, early Bronze Age) can be pre-canon in Highlander, and no stories at all can be pre-canon in Doctor Who or Hitchhiker's Guide. It would mean that WandaVision pre-canon allows only Agatha's youth in the 1600s. Granted that flashbacks are not in most series, I've personally never yet seen someone define pre-canon in this exclusive, flashback-focused way...?
Perhaps what one might want instead of restricting the definition of pre-canon to this extent is to invent a second, new category with a new, different name for this narrower, stricter slice of pre-canon content?