In the main campaign I'm currently playing in (heavily modified 1e homebrew), the DM wants people to focus on "playing personalities" rather than "playing alignments." At the same time, our equivalent of the Players Handbook, which we call the Blue Book as it's in a blue binder), does cover alignment ("Alignment is a simplistic, short-hand label used to describe a character’s or creature’s set of general moral and personal beliefs and attitudes") and it shows up in the campaign in different ways (e.g. the party might find an evil weapon).
The Blue Book describes the nine alignments, but also that they're not nine discrete categories, that alignments can change over time, and that alignment isn't a strict box that a character fits in 100% of the time, e.g.:
"These labels can be useful at times but should not be considered a complete summary of any being. Life on Dafan is not as simple as 'good vs. evil' or 'law vs. chaos' -- there are many shades of grey and much complexity. A person might be lawful and good in most respects, but chaotic under certain circumstances -- thus they would be classified as LG(NG). Politics and personalities often have a much greater influence on whom potential allies or enemies might be. Just as in any world with a long history of wars and politics, a character might find allies ... or enemies ... where they least expect them (for example, the nominal ruler of one prosperous and peaceful nation also just happens to be a Lich). You cannot boil down an entire personality to one point on the diagram. At best the point is an “average” of their beliefs and attitudes."
Plus on this campaign world (the predomininant Human culture where most adventuring happens is based on pre-classical Mycenaean Greece) there is some cultural relativism.
"There are many different cultures on Dafan, and all of them have different attitudes towards social issues which other cultures might differ on. There is no absolute rule, for example, that slavery is automatically evil. Similarly with assassins’ guilds -- in some cultures they operate 'underground,' while in other cultures they function as a respected branch of government, providing national security services and personal protection. Cultures with slavery or civil servants who are licensed-to-kill are not necessarily 'evil.' For example, a goodly person might belong to a culture where slavery is part of life; it is how you treat your slaves that is important, such that a goodly person would treat them with kindness and respect (essentially like servants on contract), while an evil person would treat them cruelly. "
Again though, our DM wants the emphasis to be on developing and playing a character's personality, and people in the 'real world' seldom fit into neat little boxes 100% of the time, so he doesn't expect characters to do so either.
Attached is the "alignment chart" from our "Blue Book," in case anyone's interested: