Following up, after testing: @DebMcIlwrath 's method works well for a feature class. It does not work for a feature service (which is what I'm trying to accomplish. I received warnings that for the selected records attachments could not be removed, but then they appeared to be removed anyways). So if you're attempting to selectively remove batches of attachments on a feature service you would have to use a bit of a workaround, like exporting the subset of features, removing their attachments, and then replacing their old version in the feature service.
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