How to restore white dress shirts from yellow to white due to bleach?

You could try soaking them in a solution of Oxy Clean powder and water.

If the shirt fabric is polyester, or a blend of polyester, and they have already been washed and dried in high heat, then the 'yellowing' might have set.

Do you think it is bleach that has discoloured the shirts, or another reason, such as perspiration, or simply because they are old?

There are millions of remedies out there and honestly I love reading about them as I remember many of them from my childhood.

Vinegar and baking soda should work. Try soaking overnight in a tub or sink. Then wash as usual.

Soak in Milton fluid for 2 hours then wash as usual

gets my curtains up brilliant white

Bluing was made for exactly this reason. Use a TINY amount in the wash-but be careful as it will dye anything in the load blue. To prevent your stuff from yellowing in the future, stop using bleach & start using white vinegar instead. You use it exactly like you use bleach, same amount & everything. There are only 2 things bleach does that vinegar doesn't: bleach yellows white fabric & bleach is toxic so it poisons you if consumed. Vinegar is inexpensive, non-toxic and biodegradable, it's also Anti-Viral, Anti-Fungal, Anti-Bacterial, & Anti-Septic.