I honestly don't think there is any "soft cap" or "bad luck protection". I think the loot table works like a slot machine and if you roll the lucky numbers 777 you win the jackpot whether its what you want or not. but the fact is i've done more content in the month of November than i did the month of September. yet still no legendary. if they are telling the truth. their bad luck protection system really doesn't feel like its working all too well.
i remember reading a blue post on the EU half of things where they said the more end game content you play the higher chances you'll have of getting a legendary. that notion kind of peeves. because i read into it if i keep feeding the slot machine coins eventually it'll let me win no matter how big or little the winnings are. If it is like a Slot machine, then a person could go the entire expansion with out seeing a legendary. where as i have seen people get back to back legendaries from doing mythic plus dungeons in the same day. i seriously don't buy into their bad luck protection bs.