But in general, one of the points of ACME is to eliminate dependence on a single provider, and prevent vendor lock-in. ACME clients should ideally support multiple ACME CAs.

For example, Caddy defaults to both LE and ZeroSSL. Users can additionally configure other CAs like Google Trust Services.

This document discusses several failure modes to consider: https://github.com/https-dev/docs/blob/master/acme-ops.md#if...

It depends. If the ACME client is configured to only use Let’s Encrypt, then the answer is yes. But the client could fall-back to Google’s CA, ZeroSSL, etc. And then there is no single point of failure.