The website https://yaytext.com/ lets you turn any plain text into stylized Unicode. What's a software program on Ubuntu or just plain Linux that will do the same? I don't like depending on websites such as YayText which tend to come and go.
Hmmm that's quite hard because of a fundamental technological limitation. It doesn't mean it is not possible, but it seems not worth for the effort.
Yaytext or similar unicode.style(a FLOSS) are online service to style text using web technologies (HTML/CSS + Web Fonts). You get HTML/CSS when you copy output but Web Fonts are hosted some where else and served by a CDN on text view. Not all web fonts are free (many are proprietary).
So it is very logical to develop a Web App for such type of apps.
If someone to develop an offline app:
For use with online/social web view, app package should have all needed font downloaded locally and also have them also served somewhere online for others to view them. Then app will use local copies for preview and generate output to use web fonts.
For use with offline app like document editors, app package should have all needed font downloaded locally and other user should have same app & fonts or embedded fonts like with PDF. Output should/could be in another format like RTF.
xfontselis remotely related (and somewhat old school X11)