# sed --version
sed (GNU sed) 4.5
Copyright (C) 2018 Free Software Foundation, Inc.

Given
\s = whitespace, [\s] should also be a whitespace.

We should get the same results if we use [\s] in place of \s

but we don't...

test 1: replace all whitespace sequences with xx using \s - OK/Works as expected;

echo 'A  BC  D' | sed -E 's/\s+/xx/g' 
AxxBCxxD

test 2: replace all whitespace sequences with xx using [\s] - fails/not as expected - should be same as test 1 output;

echo 'A  BC  C' | sed -E 's/[\s]+/xx/g'
A  BC  C

After some experimenting it seems that inside [] sed sees all \ as literal \ characters and not part of class identifiers..

echo 'A  B\C  Csstt' | sed -E 's/[\s]+/xx/g'
A  BxxC  Cxxtt