That’s the thing. It isn’t “online”, it’s print media. I can understand how some don’t make the context distinction that online follows different typography conventions than print. TBH I don’t know what, if anything, could or should be done.
AFAIK it isn’t common for on-line writing, but I do occasionally see websites with indented paragraphs. However, when people write emails they do use this writing style. Emails are different from on-line articles.
I suspect there will be more people using indents in emails.
I believe for English this is a British thing, as the colonies simply copied from the UK. The practice probably stays on longer in the colonies while the UK already moves on.
EDIT: We may need an option to turn this feature off for email in because for many sites people never email in code sniplets, and it will be highly likely that those indents are paragraph indents instead.