Nice to meet a fellow Synology user that is using advanced features. Here is what Synology tech support told me and I am disappointed that they can’t give a definitive date on status of supporting TLS-ALPN-01. Instead they are recommending unencrypted protocols which make the platform vulnerable needlessly. Here is what Synology said to me and I am in their partner product sales program.
"We are aware of the change with Let’s Encrypt, however this will not aﬀect your certiﬁcates.
There are 2 kinds of validations when applying for Let’s Encrypt certiﬁcates in DSM 6.0+: dns01 and http01 validations. You should not see any issues with your certiﬁcates.
Note: tls-sni-01 validation was supported in DSM 6.2, but Let’s Encrypt unfortunately disabled this validation on 2018/1/9 because of security problems.
We don’t have any information regarding whether TLS-ALPN-01 will be supported. I will submit a request to add this functionality to our systems to developers. I can’t currently give a timeline for its inclusion."