Secure until the government asks.

https://linecorp.com/en/security/article/35

Fair, but it's E2E by default, and only requires a (burner) email address to sign up. If you're careful, there's not much to be gleaned from the remaining data.

Doesn't LINE require a phone number or a Facebook account? Seeing as Facebook will often ask for a photo of you to verify your account before you can use it now, this doesn't seem like a fix for the anonymity issue.

You can sign up with a phone number or email address.

*Untrue. See comments below.

How? As far as I am aware, you can link an email to an account once it is created, though it doesn't really do much, but you require a phone number to create one. Indeed their website claims you need a phone number or Facebook account. I just tried registering using the Chrome extension and it required a phone number.

https://linecorp.com/en/security/article/164

It appears you are correct. Now I wonder if that's changed recently or age is catching up with me. I've not used it in over a year, but what you said is definitely the case now.