I'm pretty sure that I was the one that came with the idea for adding them to generic text widgets when I implemented that in KDE (originally to add spell checking in KMail, since spell checking in email clients wasn't a thing yet). At that time spell checking was only done in word processors. Pretty quickly every other environment, including Windows and macOS followed suite.
When was it that you added it to KMail?
Word '95 introduced the famous red squiggly underlines in 1995.
I believe KMail introduced spell-checking in 2004.