At Heroku, domain considered "unsafe" by Let's Encrypt. Google says otherwise

[Osiris]

@pdpglobalapp

Heroku support states:

"The error domain is unsafe comes up when Let's Encrypt considers allocating a cert to this domain as unsafe as noted here .

As far as I know, there is not much we(Heroku) can do here. I realize that we built this feature on top of Let's Encrypt service, but it may be worth your time to inquire with them directly about any options that might be available for domains in that situation."

Please advise. What is Let's Encrypt considering unsafe? What corrective action is needed?

From that link, Heroku states:

Let’s Encrypt checks domains against Google’s Safe Browsing API and will not issue certificates for domains considered unsafe.

However, that hasn't been the case any longer for some years now. See this blog post and the update in the "Our plan" section:

The CA's Role in Fighting Phishing and Malware - Let's Encrypt

Update: As of January 10, 2019, we no longer check domains against the Safe Browsing API.

So, ultimately, I have NO clue what Heroku is refering to.. They either have to dig deeper into their own process or have to give you the correct advice.