WP Mail: support for a plain-text version when sending a html body

[bhubbard]

Currently, WP Mail supports sending emails with HTML formatting. While this is beneficial for creating visually engaging content, it overlooks a crucial aspect of email communication: accessibility. Many users rely on plain-text emails due to limitations in their email clients, screen reader usage, or personal preference. Without a plain-text alternative, these users may encounter difficulties in accessing and comprehending the content of HTML emails.

Therefore, I propose the implementation of support for sending a plain-text version alongside the HTML body in WP Mail. This enhancement would offer several significant benefits:

Accessibility: Including a plain-text version ensures that all recipients, regardless of their email client or assistive technology, can access and understand the content of emails sent from WordPress sites.

Compliance: Providing a plain-text alternative aligns with accessibility standards such as the Web Content Accessibility Guidelines (WCAG), ensuring WordPress remains inclusive and compliant with regulatory requirements.

Improved Deliverability: Some email servers and spam filters prioritize plain-text emails over HTML emails, potentially increasing the deliverability of emails sent from WordPress sites.

User Choice: Offering both HTML and plain-text versions empowers users to choose their preferred email format based on their individual needs and preferences.

Integrating this feature into WordPress Core would enhance the platform's usability, inclusivity, and compliance with accessibility standards.