"When does something published by the WTBTS stop being the truth and turn into apostate literature?"
I've asked that question of elders and R&F JWs for years.
I never got an answer because I had a critical, fault-finding attitude and was therefore unworthy of an answer.
As best as I can determine, here's how it works:
1. The basic rule is that whatever the GB publishes is "the truth" until the GB publishes something that varies from it.
2. Then the old view is "old light" and the new view is "the truth."
3. As soon as anyone tries to use "old light" to embarrass or cast doubt on the WTBTS, that "old light" becomes "apostate literature."
4. This means that one statement published by the WTBTS can remain "the truth" while other statements in the same publication can be "old light" or even "apostate literature."
5. A special rule applies to predictions that something would happen by a specific date: When the thing predicted fails to happen by that date, the WTBTS never said that it would. Individual JWs "ran ahead of Jehovah" and to say otherwise is rebellious, independent, critical, murmuring, lying apostate thinking.
6. This explains why JWs need to attend 5 meetings a week, read every WTBTS publication as soon as it come out, and never miss an assembly.