You could try soaking it in the washing machine with a bleach solution but that may not do it if they’ve been through a dryer yellowed.

I have a soak feature on my machine. I’ll try some bleach.

There one thing I forgot to mention. This duvet cover is almost 30 years old.

Yeah -definitely soak it for a day or so but with that age, it might not help.

Link:
https://casper.com/blog/ca/en/how-to-whiten-sheets/

When white fabric is manufactured, it’s soaked in a brightener. That’s why when you have white fabric, sheets, towels, etc., and you bleacn them, they will come out white but not the bright white color that they are when they’re brand new. They’ll be white but more of a dull white. You could try to find a white fabric brightener and see if that helps.

I went to Amazon and searched for fabric brightener and I found a few things. This is one:

https://www.amazon.com/Rit-Dye-8-Ounce-Whitener-Brightener/dp/B00AQ72WWQ/ref=sr_1_7?crid=1D0WLEHG4KI8&keywords=fabric+whitener+and+brightener&qid=1648671228&sprefix=fabric+brightener+%2Caps%2C337&sr=8-7

Try soaking it in bleach.

If depends on the fabric. Look at the tag for advice about bleach. If cotton I would put it in the washer with detergent and a cup of bleach.

Soaking in bleach is not advisable.

@SnipSnip I was suggesting soaking it in a combination of bleach and water, not only bleach. Maybe detergent too?

@janbb The bleach is hard on fibers and can weaken them. So, my advice is use bleach if the fabric can handle it but don’t soak it in it; most fabrics should not be bleached. Even the cottons that have certain chemical treatments (such as no-wrinkle cotton) should not be bleached. Cotton that has any stretch to it likely has spandex and should not be bleached. Because we live in a sea of chemicals, life is not as clear cut as it was once. No so long ago cotton fabric could always handle chlorine bleach. Today many items are microfibre (which I would not even allow in my home) which should not be bleached.

Another thing to mention is that if your water has iron in it, using bleach will cause your clothes to turn yellow/orange. That is rust.

I’ve had a lot of luck adding vinegar or baking soda to my white loads. I have also added both but not sure what that would do to 30 year old fabric. I prefer to not use bleach unless it is the only thing left to my arsenal

Whites will yellow with age even IF you NEVER use them!!! Whenever I do laundry, I add about ¼ cup vinegar. It seems to keep my whites whiter, my colors from looking less faded, & I find it a GREAT way to keep the inside of my washing machine smelling fresher…nothing comes out smelling like vinegar…just fresher!!!

If all else fails, you might try bluing.

Oxi clean has something called Oxi White Revive. If you try that I would stop the wash halfway and let it sit in the water till the next day before finishing the full wash. Old yellowish stains don’t happen overnight and so it takes a bit to get them out.
If that doesn’t work there is this recommendation on how to get whites bright again but it requires several things and you may need to scrub it all. Also the example is armpit stains which comes from body oils, not natural yellowing. Though I can tell you I know dawn liquid soap does get a lot of stains out but mostly if they are some sort of oil based stain. Good luck. If any of these do help be sure to take before and after pictures. I would love to know myself which works best. I have some old car rags I wish I could wash out the stains so I know the difference between the dirty ones and the actual clean ones. Bleach only goes so far with car dirt stains.