Re-rolls have not been removed, but the way assignments are given has changed. In original RS2 tasks had specific minimum and maximum amounts for assignments. In addition to this the tasks had 'weights' which equates to the chance you have for a task to be assigned. The theory is basically that a monster has a weight from around 5 through 12 which you can consider as how many times you write that monster's name on a slip of paper and put it into a hat with all the other weighted task papers. You then draw a random paper and that's your assignment. It's simple probability and basically just means that a task with higher weight has a higher chance to be picked.
tl;dr Task amounts have been fixed and therefore are a lot lower than they were before, but the odds of getting a desirable task (AKA memevelds) is much different and better than before.
Does task weight affect how likely I am to get a task of 200 bloodvelds?
-No. Task weight only affects how likely you are to GET a specific task, once you get that task the amount you get is flat random between min and max task amount for that task.
You can read more at these wiki pages:
Click on the individual master and you'll see an entire list of tasks (which have also been corrected) as well as the weights on the right hand side of the table.
P.S Maybe sometime down the road I'll run an algorithm to print specific chances per monster on the different masters.