Yes, it's 2019 and I'm still in favour of plaintext emails. Sorry.
At worst it looks like plain text with a few _ and * characters sprinkled around, it has beautiful clean HTML output and some terminals support italic and bold fonts; win/win/win.
Pity that boat has long since sailed.
# example heading
You can add emphasis with asterisk or _underscores_.
Links can be done with  tags
And as an alternative, nothing stops you from using Markdown syntax in plaintext emails for adding structure like (nested) bullet lists.