You could, but you shouldn't have to. If you want to sign up for XYZ, you need to sign up for BigCorp, you need to add your phone number to verify your account, etc.
The "etc" is pretty important here. You can log in using Reddit, and you can create a random throwaway Reddit account without filling in any other details (no email address or phone number required).
I believe they no longer allow new accounts without an email address.
It used to be that reddit had a user creation screen that looked like you needed to input an email address, but you could actually just click "Next" to skip it.
The last time I had cause to make a reddit account, they no longer allowed this.
You're right, it looks like these days you have to fill in an email address. Though any random thing can be filled in. They will send a verification code, and on the next screen you can either fill it in or click Skip in top right. Then in the preferences, it can be removed from the account. A bit annoying for sure, but still not valid email address needed.
But it is true that at any time they could make using an email address or phone number mandatory, and then creating an Advent of Code account will be gated behind that.
Having done my own auth I get why they do it this way. LLMs are already a massive problem with AoC, I imagine an anonymous endpoint to validate solutions would be even worse.
Having done auth myself, I can also understand why auth is being externalised like this. The site was flooded with bots and scrapers long before LLMs gained relevance and adding all the CAPTCHAs and responding to the "why are you blocking my shady CGNAT ISP when I'm one of the good ones" complaints is just not worth it. Let some company with the right expertise deal with all of that bullshit.
I'd wish the site would have more login options, though. It's a tough nut to crack; pick a small, independent oauth login service not under control of a bit tech company and you're basically DDOSing their account creation page for all of December. Pick a big tech company and you're probably not gaining any new users. You can't do decentralized auth because then you're just doing authentication DDOS with extra steps.
If I didn't have a github account, I'd probably go with a throwaway reddit account to take part. Reddit doesn't really do the same type of tracking Twitter tries to do and it's probably the least privacy invasive of the bunch.
