Yes, this is correct. I made the note on the project proposal that translatewiki disabled translation to 'bbc' https://translatewiki.net/wiki/Special:Translate?language=bbc&group=core-0-mostused for unknown reason. It is weird and it didn't link to any prior discussion why translatewiki decided to use bbc-latn instead. I agree wholeheartedly that it should be changed from bbc-latn to bbc.

I believe we should do the reverse:
If a spoken language is written in (and translated into) more than 1 script, we should always specify script.

I would say those are some kind of unmigrated / historical to-dos.

That means: if there's no one using bbc-batk, we could/should simply move all of bbc translations to bbc-latn (but if someone is going to / there is/are plan(s) to translate bbc-batk, we should keep them in bbc-latn as-is).

I'll preface this response by saying that I know a very tiny bit of Indonesian, and I don't know any of the other languages I mention. So I'm not a true expert, and if any of my statements are wrong, please correct me.

That said, as far as I can see, all the localization in translatewiki is in Latin, and practically all the content in the Incubator is in Latin, too. The Batak script is used on a few pages, probably for quotations or for artistic or symbolic purposes. I can imagine that some people will write some whole pages in the Batak script, similarly to how it is in Javanese and Balinese, but not much more. So even though the Batak script is used by some people for some things, the Latin script is clearly the primary one. In such cases, it makes more sense to simply use bbc for the primary thing, as it is in other languages of Indonesia in a similar situation.

I am in contact with the Batak Toba contributors, particularly Surung Simanullang, and he's practically the only person who at this point still uses Batak script (whose font has several flaws, and we're currently trying to fix that and my Batak Toba transliterator). He did that not for artistic nor symbolic, but indeed to revive the script among younger generations (he made a few youtube videos detailing how to use the ULS, for example)

So I'm again, in favor of making the default code to be bbc, and not bbc-latn (redundant). bbc-batk is a script variation of bbc, and this is in adherence to other scripts originating from Indonesia, from ban (Balinese), jv (Javanese), su (Sundanese), etc. [A reminder that we're talking about the translatewiki convention, btw., not the site code, nor other codes]

I'm not in favor of making the default code to bbc-latn, unless you can back up your statement " If a spoken language is written in (and translated into) more than 1 script, we should always specify script. " {{cn}}
[Edit: And now I see you're spreading your claim to two other tickets. Why are you doing this to languages unrelated to you?]

The sad thing is that the written language of many indigenous language (700+) languages of Indonesia is just not existent at this point (no books, no internet literature except Wikipedia, etc). Many languages are spoken languages only, and in the case of Batak (in general) and Batak Toba, the original script have not been used for hundreds of years, and by this point, the majority of them can't read it anymore.

In T341946#9018771, @Bennylin wrote:

The original script have not been used for hundreds of years, and by this point, the majority of them can't read it anymore.

I think either using bbc-latn or moving to bbc are all fine based on this statement.