Copy/pasta from the Reddit post, to explain in more detail. Feel free to Tweet me @JagexRowley (especially interested in KBD kill counts at time of receiving Last Riders this week, doubly so if it's >2,000 - i.e you already had most of those kills prior to this update).
Re: bad luck mitigation - this seems as good a post as any to explain this on.
The goal was to respect personal achievements, but to lend a helping hand to a niche of players that are statistical outliers (especially in some cases extreme outliers). We arrived at this after taking plenty of suggestions and ideas from players, both with and without MQC.
The general rule is that you have to put in a solid effort before the mitigation even starts to kick in - this being a threshold of kills equal to the base drop chance (e.g. for 1/100 drops, you need 100 kills first). The drop chance then increases per kill until at double this threshold, a drop becomes guaranteed.
For mobs with a series of lore drops, the threshold increases for each new drop in the sequence. e.g. with rippers, the thresholds are at 100 kills for book 1, 300 for book 2, ... up to 1,100 kills before mitigation kicks in for the final book. The majority of players will have received all six books by 600 kills (give or take a certain amount; I got all mine in ~650 kills, for instance), so will barely have triggered any mitigation (if any).
These values mean that anyone who receives any mitigation has typically already put in MORE effort (and in some cases FAR more effort) than the average player who has gotten these drops 'naturally' (if you will), so doesn't make MQC easier to get.
If you can kill 1,000 KBs, you can theoretically kill 10,000, but you shouldn't have to just for a lore drop. You've proven you are capable, so your success should no longer be determined by luck (especially not bad luck).
For Last Riders specifically: KBD masses are unchanged - it remains a straight 1/1,000 drop chance, but if it drops, anyone in the mass who doesn't have the book can pick it up. Masses are still worthwhile as a faster way to increase your kill count (and still have a chance at the group drop), but once you have over 1,000 kills, the mitigation starts to kick in. At 2,000 kills, head off for a solo kill.
We've only added it for lore drops with a drop chance of 1/100 or worse. For court summons from monster drops, however, you're still much better off getting them from caskets. Angof's ring, too, already has mitigation in the form of the T80 crystal armour (with a full set the drop chance is 1/10), so it didn't need any further mitigation adding.
Finally, the lore drops have been decoupled from any rare drops (e.g. dragon rider equipment from KBD). They 'may' still drop at the same time due to random chance, but not 100% of the time as is the case before this patch.
That should answer most questions about this, but I'll keep an eye out for any more. Also going to keep an eye on how this is received (the threshold values could be tweaked, for instance). The intent would be that future lore drops of 1/100 or worse would have mitigation at launch or that we wouldn't have lore drops with such a low chance. Time will tell on that one.