Well so many question that, and i am sure i won't get many answer in the manga but whatever
think like how a fucking immortal being was not already scout or "kill" before.
i mean it's a KNOW fact xp accumulate, because a newby in a guild know it, now every immortal/long live being would being EXTREMELY powerful (the fact a dragon is weak even if they kill a lot even after thousand of years)
And the village (and probably some traveler) know she is immortal
Now you gonna have a pretty good explanation for the fact that nobody did care for 300 years
And i got so many other question but this one is probably the biggest, yeah i know it's a Slice of Life, but i am quite sure SoL =/= pass for having stupid world building
Edit : yeah i still can't pass on the fact everyone know she immortal, kill slime for a living, know how much a slime give you in XP and still act surprised at the fact she is lvl99
i mean the fucking newby at the guild did calculate the thing super quickly, now all i can deduct from the world building is, all the population of the village is mentally limited
AND yeah i can bet that slime farm do not exist in a world like that even if would be a really good solution