Yes, sending plain-text versions of your emails remains important. One specific reason is to ensure compatibility across all email clients. While HTML emails might render well on most platforms, some older email clients or security settings may not display them correctly, leading to a loss of message impact. Providing a plain-text alternative guarantees that your content is readable and understandable, regardless of the recipient's email setup. This approach also caters to individuals who prefer simple, text-only emails or use assistive technologies, improving overall accessibility and engagement.
Sending both an HTML and Plain text version of an email is called multi-part MIME, and it is considered best practice by most Mailbox Providers (MBPs). Multi-part MIME is also one of those things where anecdotal evidence suggests that it is being considered a positive data point when assigning reputation to a mail stream. There is no harm in sending multi-part MIME email, but there may be harm in not sending it ;-) In short, yes, you should still send plain text versions of email.