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Are there any pre-eternal-September warriors still sticking Emily-Post-like to their 1992-era netiquette standards in the face of modernity?Yes, and one time it comes into play is when you're dealing with someone else who uses those conventions. IME, people still doing that are very sharp. And if they're doing it, you want to do it in kind, both because it can be more effective, and also to let them know you're clueful enough to know that style of email-fu.
But I also adapt to other people and organizations. For example, if some business person seems to keep the past history quoted, and also might be forwarding it within their bureaucracy to people who also expect that, then use that style. (And maybe also repair it to not drop history, such as when they respond to an old email, or they send two responses to the same people before anyone responds, or there's race conditions or oops/indifference when multiple people are responding. Not everyone has given any thought to how this works; they just start typing where their corporate-drone webmail or app left the cursor and quoted everything for them.)
When I have the luxury of setting an organization's conventions, I try to get people to capture all non-code institutional knowledge in a wiki and issue-tracker, and to not lean on ephemeral/silo'd email for the "history" of something, nor to create new silos in a dozen different SaaSes. (This also occasionally involves copy&paste of an email with important info into a wiki page, as the easiest way to leave some important info where whomever later needs it will see it.)