As I have announced, acme.sh was sponsored and acquired by apilayer a year ago: https://twitter.com/neilpangxa/status/1222792801835872256
ZeroSSL is an ACME compatible free CA by apilayer.
Acme.sh will change default CA, but it's still open and free. Users are still free to choose to use any ACME compatible CAs.
ZeroSSL is almost the same as Letsencrypt: support unlimited 90days certs, including wildcard certs. Full ACME compatible.
Moreover, as letsencrypt is going to change the crossing-signed root, ZeroSSL's setigo root will have a better compatibility than letsencrypt's.
There is also a 6 months period for the users to make choices.
And, the users can select back to use letsencrypt anytime.
So, I think this change won't hurt the users.
As the name implies, acme.sh will always stick to RFC8555 ACME protocol.
It will always keep open and free.
Thanks.
-Neil