All text in Taskwarrior is UTF8, which means any Unicode characters can be entered and stored. Here is a demo:

Both the U+NNNN and \\uNNNN specifiers are supported, but it is usually simpler to use the first, which does not require the backslashes to be escaped in shells and scripts.

Note that the font you use in your terminal determines whether those characters are rendered, so it is possible to enter characters for which there is no glyph.