There is little chance the Standard is gonna get updated again, especially since CDR (the Common Lisp Directory Repository) kinda stopped. That said there are a few libraries that are kinda considered to be the default go-tos for various needs:

CL-PPCRE for regular expressions.

Bordeaux-threads for threads.

Alexandria for various miscellaneous stuff.

Trivia for pattern matching.

CFFI for, well, FFI.

And Series for functional iterative sequence-like data structures.

There's also ASDF and UIOP, but I'm not sure whether or not they're part of the Standard.

Too bad cl-ppcre is extremely slow in my experience, but that's what we have. ASDF and UIOP are the same as the rest, de facto standards, but not ANSI nor CLtL.

I would also add iterate to the "must haves". All noobs should constantly look at the CLHS (available for dash, zeal and emacs too!), https://lispcookbook.github.io/cl-cookbook/ and https://github.com/CodyReichert/awesome-cl, anyway.

Truly, someone could tie himself to SBCL and its extensions and get a much more modern environment, but I think it's worth targeting ECL and CCL in addition.

there's a much faster regex library: https://github.com/telekons/one-more-re-nightmare

It's not yet operational, sadly. Missing ^ and $ isn't a small thing.

ASDF is one of the symptoms of what is wrong about CL. You have this thing that is not really sure whether it is image based or source based and has a bunch of global state while reading the source. Solution to that is ASDF, which is a ridiculously hairy and complex thing.