Rav Yisroel Belsky ztl writes
In olden times, the very greatest medakdekim b’mitzvos, those Jews who
were exceedingly meticulous in their observance of mitzvos, were
careful never to sit on cushions or mattresses made from shredded
fibers. It was a very big chumrah for two reasons. Firstly, there was
only a very slight possibility that these cushions and mattresses were
flexible enough to wrap around a person and give him warmth. It is
permissible to sit on stiff objects, even if they contain shaatnez.
Secondly, such cushions and mattresses only contain linen and wool
fibers, not woven shaatnez, and the halachah follows the other
Rishonim, that a mixture of linen and wool fibers is only assur
miderabbanan. Thus, these pious Jews acted in deference to the lone
opinion of the Rambam, and were machmir even with stiff cushions and
mattresses, that could only vaguely be seen as providing warmth and
protection in the manner of a piece of fabric that is worn. For these
reasons, such cushions were seen by the great majority of Klal Yisroel
as entirely permissible, but were avoided by the scrupulous.
The situation with cushion and mattress stuffing today is different
than it was many years ago. This is due to the preponderance of cotton
and synthetic fibers in the textile industry. By combining the wool
and linen with a third substance (min shlishi) that is greater in
proportion to either the wool or the linen, any shaatnez that might be
present is nullified. Once this is done, the fibers can even be spun
and woven into a fabric. Halachah permits shaatnez to be nullified
even if the wool and linen combination had been created first, before
the introduction of the cotton or synthetic fibers.
The fibers used for stuffing cushions are made entirely from offcuts
from the garment and upholstery industries. They are sold in bulk to
shredding factories, who make the stuffing that is used in anything
from baseball gloves to shoulder pads to upholstery. These shredding
factories purchase whatever scrap material is available. In today’s
market, most of the fabrics are synthetic and wool, whilst linen
accounts for only a very small percentage of the total. It is always
the case that the crushed fiber fillings contain at least enough
synthetics to nullify the linen, and often there is enough to nullify
even the wool. This means that today, there is no reason at all to be
concerned about shaatnez in cushions and mattresses.