Is it because they can't tell if it's beneficial to the protocol or actually a negative?
The former.
Thank you, is there a reason why tagamet isn't recommended anymore?
Honestly, the benefits just can't be nailed down. If you check this site for antacid, you'll find a ton of debates about whether it's good or bad, but in the end, it's all pretty much just educated guessing.
The PPEH says that the benefits or potential of an antacid "cannot be established." Plus, Vizzy, who cites Dr. PN from the PPH, mentions that some nitric oxide production from the reaction between stomach acid and nitrite
might help lower blood pressure and
might speed up unconsciousness.
Basically raising the pH of the stomach (lowering the acidity) decreases the SN converting into Nitric oxide which is a vasodilator that will drop your blood pressure drastically resulting in unconsciousness. This is the main and fastest mechanism that SN knocks you out and antacids can slow this process resulting in you staying conscious longer. Most people think SN knocks you out by lowering oxygen levels in your tissues and yes that would eventually lead to unconsciousness, but the vasodilator effect is much quicker and is why people who take SN can collapse at any time without warning so it's best to be laying down or sitting after you take it.
Here was Dr. PN's direct words as to why it was taken out from the protocol though:
It sounds promising in theory, but when it comes to measurable differences—like whether it shortens unconsciousness by one, two, three, or even five minutes or any at all—it's all uncertain. That's why the protocol states that its benefits or potentiation cannot be established.
That said, I don't believe using it would harm the protocol if someone chooses to do so.
I hope this helps and doesn't just add more confusion around it. With whatever you decide, I hope you find everything you are looking for.