Pseudo-deterministic number generator
Python 2, 23 22 bytes

Thanks @xnor for figuring out a really cool way to convert the seed to -1 or -2, which saved 1 byte.


lambda s,n:`n+9`[2/~s]


Try it online!

Input: The seed s (non-negative) and an index n (positive).
Output: The element at the nth index (the sequence is one-indexed)

If the seed is positive, the sequence is 0123456789 repeated infinitely.
If the seed is 0, the sequence is: 1111111111 2222222222 ... 9999999999 0000000000 (where each digit repeats 10 times) repeated infinitely.

How


`n+9` creates a string from a number that has at least 2 digits.
2/~s evaluates to -2 if s is 0, or -1 if s is positive
Thus `n+9`[2/~s] takes the last digit (unit digit) of \$n+9\$ if s is positive, or the second to last digit (ten digit) if s is 0.