Even though Timmerman's were "educated guesses", they have stood the test of time. I think many of us have them drilled into our souls.
I would guess, at least for the next few years, no one can really say you're "wrong" for using either. HyTEC is, by Timmerman's own admission, "better", but until and unless the ABR decides to release a statement about "ABR-sanctioned constraints", I can't imagine you would fail for using the Timmerman constraints (which I will probably fall back on myself).
It would be great if the ABR took official stances on things when fundamental parts of our practices are changing. This is a good example, the AJCC 9th edition coming out piecemeal is another, there are many other small examples.
Even for clinical writtens this came up. I don't want the ABR to do a no-knock raid on my house with this, so keeping things vague - I think many of us who took clinical writtens over the last couple years saw the question about options for definitive NSCLC treatment. There were multiple "correct"/"reasonable" options (especially if you read the ASCO lung guidelines just published).
What was the point of that question? Who wrote it, and what were they looking for? Why is this a black-box process? Will one examiner "fail" you for using Timmerman constraints, and another examiner will be OK with it?
ABR, in the background: "Don't worry, Angoff!"