The fix is LGTM.

Some users may not be sure what "the whole empty string" is.
I suggest explaining in more detail, and no need to use "whole" for "empty string".

\B used to be unable to match empty string. Now it can match, this behavior is consistent with mainstream languages.

I know why you use "whole", the \B description:

\B Matches the empty string, but only when it ...

You want to distinguish from "empty string" here.
Let me think about how to describe it more clearly.

It is still difficult to avoid contradiction with "\B Matches the empty string, but only when it ...".

Yeah, "the empty string" means two different things there. The docs you're removing don't do a very good job either. Should be disambiguated somehow, for example

\B now matches if the input string is empty.

How about this:

\B used to be unable to match 0-length string ""/b"". Now it can match, this behavior is consistent with mainstream languages.

\B now matches if the input string is empty.

@Alcaro uses the word "input", it looks fine. Candidate:

\B now matches empty input string ""/b"", this behavior is consistent with RE implementations in mainstream programming languages.

The previous test: #124130 (comment)
After that, I tested ASCII mode for "a", Unicode mode for "ю".
The results are same except in Perl v5.40.0.

Tested with: regex module, openjdk, .net, rust, ruby, php, pcre2.
javascript/golang only support ASCII word (for \w\W\b\B), even in Unicode mode.

It seems Perl v5.40.0 has bug in (\B + Unicode_mode).
Given this, it's not recommended to mention Perl in doc.

Save it to file perl.pl, and run: perl perl.pl

print "Perl version: $]\n\n";

# test \b in ASCII mode
my $n = () = ( "" =~ /\b/g );
print "\\b \"\"  matches: $n\n";

my $n = () = ( "a" =~ /\b/g );
print "\\b \"a\" matches: $n\n";

my $n = () = ( "=" =~ /\b/g );
print "\\b \"=\" matches: $n\n";

print "~~~~~~~~~~~~\n";

# test \B in ASCII mode
my $n = () = ( "" =~ /\B/g );
print "\\B \"\"  matches: $n\n";

my $n = () = ( "a" =~ /\B/g );
print "\\B \"a\" matches: $n\n";

my $n = () = ( "=" =~ /\B/g );
print "\\B \"=\" matches: $n\n";

print "\nxxxx  ASCII mode above / Unicode mode below xxxx\n\n";

# test \b in Unicode mode
my $n = () = ( "" =~ /\b/gu );
print "\\b \"\"   matches: $n\n";

my $n = () = ( "ю" =~ /\b/gu );
print "\\b \"ю\" matches: $n\n";

my $n = () = ( "=" =~ /\b/gu );
print "\\b \"=\"  matches: $n\n";

print "~~~~~~~~~~~~\n";

# test \B in Unicode mode
my $n = () = ( "" =~ /\B/gu );
print "\\B \"\"   matches: $n\n";

my $n = () = ( "ю" =~ /\B/gu );
print "\\B \"ю\" matches: $n  <- other REs get 0, it seems a bug.\n";

my $n = () = ( "=" =~ /\B/gu );
print "\\B \"=\"  matches: $n\n";

# explanation
print "\n(\\B + Unicode_mode) behaves inconsistently at the begin/end of a string:\n";

print "\n/^\\B/gu NOT IN \"ю\":\n";
my $n = () = ( "ю" =~ /^\B/gu );
print "matches: $n\n";

print "\n/\\B\$/gu IN \"ю\":\n";
my $n = () = ( "ю" =~ /\B$/gu );
print "matches: $n\n";

Script output:

Perl version: 5.040000

\b ""  matches: 0
\b "a" matches: 2
\b "=" matches: 0
~~~~~~~~~~~~
\B ""  matches: 1
\B "a" matches: 0
\B "=" matches: 2

xxxx  ASCII mode above / Unicode mode below xxxx

\b ""   matches: 0
\b "ю"  matches: 2
\b "="  matches: 0
~~~~~~~~~~~~
\B ""   matches: 1
\B "ю"  matches: 1  <- other REs get 0, it seems a bug.
\B "="  matches: 2

(\B + Unicode_mode) behaves inconsistently at the begin/end of a string:

/^\B/gu NOT IN "ю":
matches: 0

/\B$/gu IN "ю":
matches: 1

I'm sorry, Perl doesn't have (\B+Unicode_mode) bug.
If add a line use utf8; at the beginning of Perl script, it works as expected.
ref: https://perldoc.perl.org/utf8