[xmasons]

Currently the new feature (bbPress 2.5) of Subscribe to Forum returns plain text email. Perhaps this was a conscious decision to implement it in that way. I’d implore either changing the content-type default to TEXT/HTML or be given an option within the admin settings.

Typically you would only need to add the following:

add_filter('wp_mail_content_type',create_function('', 'return "text/html";'));

…before @wp_mail

[Stephen Edgar]

I didn’t make it but I expect it was a decision made not an option for the broadest compatibility.

Decisions, not Options https://wordpress.org/about/philosophy/
When making decisions these are the users we consider first. A great example of this consideration is software options. Every time you give a user an option, you are asking them to make a decision. When a user doesn’t care or understand the option this ultimately leads to frustration. As developers we sometimes feel that providing options for everything is a good thing, you can never have too many choices, right? Ultimately these choices end up being technical ones, choices that the average end user has no interest in. It’s our duty as developers to make smart design decisions and avoid putting the weight of technical choices on our end users.

Though as you point out if you want HTML emails you can enable this pretty easily.