On the downside, even today, Tcl can't handle characters outside the basic multi-lingual plane. It only does UCS-2, it can't handle UTF16 surrogate pairs. If you convert an astral-plane codepoint, such as some popular emoji, from UTF8, TCL will convert each UTF8 byte into a separate unicode codepoint. There are similar catches all over, it's just not practical to deal with non-BMP codepoints in TCL, even just to round-trip them.

Rereading OP's comment, I misread it a bit and responded to the wrong part. You're absolutely right, progress on Unicode in Tcl stalled out after the low-hanging fruit of UCS-2 was achieved.

Well, it's not about "supports unicode" as much as it's about the level of support. IIRC, I've heard that Tcl had fairly good unicode support (at least for the time), but I have no idea how it compares to some contemporary versions of languages, or Perl 6 specifically.