Say what now? Pretty sure sha-1 support for certs is much older than android 7.
True!
Now, for most (perhaps even all other?) Certificate Authorities that doesn't prove much, because after all you could manually cause issuance of anything. But the fun thing about Let's Encrypt is they've insisted almost to a fault upon only using their ACME API. After all, if you really believe in everybody else using automation it's hypocritical to have manual systems for your own stuff.
[ I say almost to a fault because there can be situations where for compliance you need to demonstrate something and it seemed (I may be wrong and Seth will correct me) as though Let's Encrypt always chose to just let nature take its course instead. Want to see an expired certificate? We'll put one of our auto-generated certificates on a server, and then just wait ninety days for it to expire. ]
So as a result helloworld.letsencrypt.org isn't just a proof that they could issue such certificates, it means logically that's what all the certificates looked like at that time.
At least I know that there is no UI for manual issuance (or for manually resetting a rate limit condition), so there's no person you can call up and try to harangue/social engineer into doing these things for you.