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Yes, there are a number of third-party utility packages (and many, many e.g. personal custom bash prompts) that check the value of the $VIRTUAL_ENV variable to detect whether one is currently active, and display its name. Unless there's an overriding reason, it would be nice to not require changing all of this code.
Certainly not all third-party virtualenv-related tools will be compatible with pyvenv unchanged; for instance tools that create envs will need to use the updated pyvenv API. But there is a lot of code using $VIRTUAL_ENV that won't require any other changes, if we can keep using the same env var.