Even just putting half the key on paper and not putting the rest could make brute-forcing the rest feasible. Even knowing just 1 bit makes brute-forcing 2x as easy. 8 bits? 256x easier, etc.
> To unlock the secret via Shamir's secret sharing, a minimum number of shares are needed. This is called the threshold, and is used to denote the minimum number of shares needed to unlock the secret. An adversary who discovers any number of shares less than the threshold will not have any additional information about the secured secret-- this is called perfect secrecy. In this sense, SSS is a generalisation of the one-time pad (which is effectively SSS with a two-share threshold and two shares in total).
[1]: https://pimbook.org/