This is the webnovel.
https://www.novelupdates.com/series/tate-no-yuusha-no-nariagari/
I mean, I guess you can say that, though webnovels have a different writing and shit, they are close to light novels, but it still is called canon, its just that there will be no confirmation if the writer changed the story or not. Not to take literally, but there are some that are literally the same, its just in online format, so the one I've been reading is quite unconfirmed. But another word is that, the manga also changed before it took itself to the 30s prolly so everything can actually change midway.
At this point, even if I was a novel reader, I dont know what to expect. The manga basically went and had its own other half plot.