Every time there's a big iOS update–either a major yearly release or a particularly feature-packed point release like iOS 15.4–you'll find users complaining that their battery doesn't last as long as it used to. On social media, users will complain of significant battery drops just hours after the update.
Those who follow this closely have maintained that this is to be expected. The operating system is performing background operations, optimizing all sorts of data, and sometimes doing stuff like re-scanning your photos with new machine learning algorithms, and this increases battery drain. Not to mention that battery management is handled by sophisticated algorithms that may need several charge-and-drain cycles to adapt.
In a rare official comment on the matter, the Apple Support Twitter account confirmed this to be true.
