To simplify the expression 2021-[(26-15)^2-(2^3.3^2+28)], we need to follow the order of operations (PEMDAS/BODMAS).

First, let's simplify the innermost parentheses:
26 - 15 = 11

Next, let's simplify the exponentiation:
2^3 = 8
3^2 = 9

Now, let's substitute these values back into the expression:
2021 - [(11)^2 - (8 * 9 + 28)]

Next, let's simplify the multiplication and addition inside the parentheses:
8 * 9 = 72
72 + 28 = 100

Now, let's substitute this value back into the expression:
2021 - [(11)^2 - 100]

Next, let's simplify the exponentiation:
(11)^2 = 121

Now, let's substitute this value back into the expression:
2021 - [121 - 100]

Next, let's simplify the subtraction inside the parentheses:
121 - 100 = 21

Now, let's substitute this value back into the expression:
2021 - 21

Finally, let's perform the subtraction:
2021 - 21 = 2000

Therefore, 2021-[(26-15)^2-(2^3.3^2+28)] simplifies to 2000.