Our grandmothers' habit of boiling whites made sense with the detergents and soaps of their time. Today's detergents are designed to work at low temperatures. That does not make temperature irrelevant, it just changes its role.
For duvet covers and pillowcases, 60 °C is a sensible threshold. It is enough to deal with house dust mites and many bacteria, which matters especially for people with allergies. For coloured and dark items, 40 °C is usually a better balance for protecting colour and fibre. As for 90 °C, at home it is mostly unnecessary. It tires the fibres, uses energy and increases the risk of shrinkage.
One more thing: in good-quality products, dyes are usually chemically bonded to the fibre and cope with 60 °C easily. But if a cheap product bleeds colour in its first hot wash, the problem is not the temperature but the dye itself. The wash symbol on the label is a promise from the manufacturer. Following it protects both the product and your guarantee.