Dwarves aren't cool anymore

[JusticeZero]

I believe you can measure (at least in a threshold presence style of measurement) Good and Evil and Law and Chaos because there are spells that detect those things. I believe that a game might very well have a clear definition of Good and Evil and Law and Chaos. I do not necessarily believe, however, that Good is always synonymous with good, Evil with evil, Law with law, Chaos with chaos.

We are in full agreement on this.

However, we are discussing capital E Evil, Capital G Good, etc — those things which can be measured with reliability but which have questionable validity.

There is literally no reason to discuss small g good, small e evil, et cetera. These things simply do. not. matter. If something is Always Evil, it can be extremely good so long as it remains Evil, so the only discussion that matters is "How does the relevant universe's physics derive its measurement of Good or Evil?" And philosophical discussion misses the point, because it's simply too refined and nuanced of a tool.

"Detect Evil detects the presence, above a set threshold, of the quality the setting in which it is cast, defines Evil. That definition of Evil may not be the same as local or regional definitions of Evil and/or evil, where "evil" is defined by the morality of the people playing the game that contains said spell."

I don't even agree with that. Casting certain magical effects is known to alter the alignment of the caster in absence of any external moral considerations, for example. We can certainly postulate that the game universe follows theories of alignment to which we as players do not subscribe, in part because the existence of a universal, non-relative measuring system is in defiance of many established theories of ethics we might use. Since the tropes violate our ethics in one regard, we can certainly picture that it violates it in others. While it might approximate our ethics, it fails to mirror it; it is not sufficiently valid.

Is this ("must perform Good acts at every point") game-supported?

Consider a demon who performs evil acts, or a depraved angel. If they retain their status and original detectible alignment then they are highlighting that the test is reliable but not valid. They can do whatever they want; their alignment cannot change.
If they will lose their status and alignment, then they will tend to random walk away from "Absolute Alignment". The races of Angels and Demons lose their status of "Always Good/Evil".