https://github.com/tc39/proposal-bigint/issues/208 got consensus at the July 2019 TC39 meeting. That means 08n is now no longer allowed and instead should throw a SyntaxError.

I can take this next week; will be out of office until Wednesday. Please feel free to steal if appropriate.

Doc update needed (at least) here: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Lexical_grammar#Numeric_literals

Pushed by archaeopteryx@coole-files.de:
https://hg.mozilla.org/integration/autoland/rev/b29469828c33
Disallow BigInt literals for Annex-B non-octal digits r=anba

https://hg.mozilla.org/mozilla-central/rev/b29469828c33

Posted site compatibility note: https://www.fxsitecompat.dev/en-CA/docs/2019/bigint-literals-for-non-octal-digits-are-no-longer-allowed/

I've added this to the Fx 70 for developers release notes: https://developer.mozilla.org/en-US/docs/Mozilla/Firefox/Releases/70#JavaScript
The reference pages that describes numeric literals has been updated to talk about this, too: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Lexical_grammar#BigInt_literal

:anba, could you double check if these doc updates are correct?

(In reply to Florian Scholz [:fscholz] (MDN) from comment #7)

I've added this to the Fx 70 for developers release notes: [...]

Thanks!

:anba, could you double check if these doc updates are correct?

https://developer.mozilla.org/en-US/docs/Mozilla/Firefox/Releases/70#JavaScript:

• Nit: Contains "legacy legacy".
• Even before this bug, legacy octal numbers weren't allowed in BigInt literals, i.e. 0755n from the example code was always a SyntaxError. But before this bug, 08n was allowed and treated the same as 8n. The spec change made this an error.

So maybe use something along the lines:

Per the latest ECMAScript specification, a leading zero is now never allowed for BigInt literals, making 08n and 09n invalid similar to the existing error when legacy octal numbers like 07n are used.

Aha! Then this change is even more subtle than I thought.
Thank you so much for clarifying this, :anba! I've updated the release docs.