I noticed that I had not quite finished the work of commit 854ed8a, which removed the test for support for POSIX character classes, as that is now guaranteed.
In ispell-set-spellchecker-params, there is code that used to be run conditionally on support for POSIX character classes, which sets all the CASECHARS and NOT-CASECHARS entries for built-in dictionaries to [[:alpha:]] and [^[:alpha:]] respectively.
There is no point doing this unconditionally, so instead, put these character classes directly into the initial values used in ispell-dictionary-base-alist. This change also makes the variable's initialization easier to read.
The attached patch makes these changes.