You could easily do an ad hoc form of this by just posting an SHA-512 hash of your predictions. This doesn’t have an integrated method to decrypt the prediction after n years, but you can publicly precommit to reveal your prediction after n years.
If you’re worried about this revealing your prediction early via a brute force attack, you can append a random sequence of tokens to your prediction before hashing it.
Gwern has a fantastic overview of time-lock encryption methods.
A compute-hard real-time in-browser solution that doesn't rely on exotic encryption appears infeasible. (You'd need a GPU, and hours/days worth of compute for years of locking). For LW, perhaps threshold aggregate time-lock encryption would suffice (though vulnerable to collusion/bribery attacks, as noted by Gwern).
I agree with Quintin Pope, a public hash is simple and effective.
Set up two bitcoin wallets, transfer funds from one to the other, and put your hash in the message field.
The bitcoin blockchain is both immutable and public, making it an ideal medium for sealed predictions. While the LW servers might be compromised, there are game-theoretic guarantees that the blockchain won't be.
For the less cryptographically inclined, or those predicting the failure of computing technology, there is always the old school method: write your prediction on a peace of paper, literally seal it in an envelope, and mail it to yourself. The postal marking they put over the stamp includes the date.
Many people in our community claim to have ideas for how to build AGI, or other things, that they deem infohazardous and so don't want to publish. It would be great if they could publicly register these ideas in an encrypted way, so that later when their predictions come true they can reveal the key and everyone can see that they called it and give them epistemic credit accordingly.
I know this is possible in principle, e.g. by using PGP and posting encrypted messages on your LW shortform and then later revealing the key.
But it would be nice if this was a convenient, hassle-free feature embedded in LW, for example.
Also: Is this a bad idea for some reason? Is the privacy not as secure as I think, such that people would be hesistant to make even these encrypted predictions? (I guess there is the matter of how to securely store the key...) Is there a way to make a prediction that will automatically be decrypted after N years?