Is it possible for Bob's citizenship be revoked (stateless)? i.e.
denying bob a passport?
No. Since Bob was born in the US, he was automatically a US citizen at birth, due to both the constitution (the Citizenship Clause of the 14th Amendment) and statute (8 USC 1401(a)). It's not possible for him to lose citizenship under any circumstances, unless he performs some "potentially expatriating act" with the intention of relinquishing US citizenship, as provided in 8 USC 1481.
There is one exception, which is if Bob was born to a foreign diplomat with full diplomatic immunity, in which case Bob would not have been a US citizen at birth. It is possible that Bob might have been able to get a US passport by mistake, and they would later deny it, giving the appearance of "revoking citizenship", even though the legal theory is that he never had it. This was the situation of Hoda Muthana, who was born in the US and had obtained US passports, but she was later denied a US passport and denied entry to the US, after it was discovered that she was born to a foreign diplomat. (There is some dispute over whether her father still had diplomatic immunity when she was born; I will not go into that.) She was deemed to have never been a US citizen, and the passports she got were deemed to have been issued in error. Since she never technically had citizenship, it was not technically "revoked".
Also, in the vast majority of cases, a child born to foreigners will inherit at least one of the parents' nationalities, according to those countries' laws. And this would be especially true in the case of a child of the country's diplomats. So even if the child did not have US citizenship at birth due to being born to a parent with diplomatic immunity, the child would still not be "stateless".
Does Bob's parents becoming US citizens confer citizenship beyond
"birthright"? Or do the parents have to be US citizens at the time of
birth?
No. But if one of the parents was a foreign diplomat with full diplomatic immunity, and the other parent was a US citizen at the time of Bob's birth, then whether Bob has US citizenship at birth depends on whether the US citizen parent meets the conditions for transmitting US citizenship to a child born abroad. This is according to former 7 FAM 1116.2-2(d)(4) from a 1995 edition of the Foreign Affairs Manual (page 8 of this PDF) and chapter 1.A of the 2005 Digest of United States Practice in International Law (page 1, on 33rd page of this PDF).
If Bob is no longer a US Citizen (Stateless), is he subject to
deportation? Where would he be sent?
If Bob was not a US citizen at birth due to being born to a parent with full diplomatic immunity, he is eligible to register to become a US permanent resident (i.e. green card holder) if he has maintained residence in the US since birth. As a green card holder, he would not be subject to deportation for lack of status, but he can of course still be deported for certain other things (e.g. crimes). Since we established that Bob is not stateless, he can be deported to the country of his nationality.
If Bob was a US citizen who later lost US citizenship by performing one of the potentially expatriating acts with the intention of relinquishing US citizenship, this would generally have happened outside the US, so deportation would generally not come up. In particular, renunciation of US citizenship can only be done in front of a consular officer abroad. Other potentially expatriating acts would also most likely happen abroad.