Euphoria from oxygen deprivation Plane &amp; Pilot have an article saying:


  Altitude-chamber tests have shown that as oxygen deprivation increases, some victims experience a sense of increasing well-being, even euphoria, while they’re losing the ability to function in a thoughtful, coordinated manner.


But it's not too clear from that article how many people experience euphoria from oxygen deprivation. I'm pretty sure the loss of cognitive function is 100%, eventually, even though the dose/concentration might vary with the subject. But does everyone experience euphoria before that happens or does it happen just to some individuals with a certain vulnerability? This video suggests not everyone experiences euphoria, and if that's so what's the prevalence?

Optionally (i.e. if it's easy to answer based on the same source(s) for my main question), are there any proposed/studied pathways by which this euphoric consequence of oxygen deprivation might come about? And is it relevant to anything else, say to the "choking game" that some children (unfortunately) practice, or to asphyxiophilia/hypoxyphilia?
Short answer
Given the fact that literature on hypoxia consistently report euphoria as a symptom, my conclusion is that the two are correlated strongly and is not confined to a small subset of susceptible people.
Background
Personally I know only of the effects of oxygen on the central nervous system under conditions of increased oxygen pressure, as for example encountered by scuba divers at depths greater than standard recreational diving. In that case it is called oxygen intoxication or oxygen toxicity (e.g., Chawla &amp; Lavania, 2001), characterized by e.g.:

visual changes
tinnitus
irritability, anxiety, confusion
vertigo.

Hypoxia (Asmaro, Mayall &amp; Ferguson, 2013) can indeed lead to

Euphoria and feelings of well-being
Acute anxiety, irritability &amp; confusion
confusion, loss of consciousness.

The authors however only qualitatively analyzed their group of test subjects on these subjective mind states and describe quantitative statistics only on psychophysical tests. The latter were indeed significant, showing that hypoxia leads to changes in performance on a variety of tasks in most, and probably all subjects.
To proceed to your question - hard numbers on the prevalence of feelings of euphoria are difficult to find, yet credible reports that mention it as a symptom are abundant. For example, the self-asphyxial risk-taking behavior (SARTB) you mention is known to lead to euphoria and is thought to be one of the motivations behind the 'game' (Griffiths, 2015; Busse et al., 2015).
The fact that hypoxia is a general term for a range of blood oxygenation levels, and the complexity of the cerebral cortex, variability in susceptibility is to be expected. For example, hyperoxic effects depend strongly on the altitude (i.e., partial oxygen pressure) Chawla &amp; Lavania, 2001).
References
- Asmaro, Mayall &amp; Ferguson, 2013
- Busse et al., Arch Dis Child (2015); 100(12): 1106–14
- Chawla &amp; Lavania, Med J Armed Forces India (2001); 57(2): 131–3
- Griffiths, Education and Health (2015); 33(3): 54-6