How to recover missing photos after macOS 15.4.1 update?

Well, as I said, "if pictures since a particular date are no longer visible, Photos may have switched to using an older library."

Try using the Choose Library dialog. To get there, Quit out of Photos entirely, then launch it again with the Option key held down. That should give you a dialog like this:

In this example, Photos saw two libraries on mounted volumes. It selected the current System Photo Library, highlighted it in green, and showed its path below the list. It lives in the default location, the Pictures folder in my home folder. If I click on the other library, I will see its location, boringly the same.

If you try that exercise, do you see several libraries? If so, try opening them to see if one of them contains the missing photos. Whichever library you open last will be opened automatically the next time you start Photos.

If you don't find another library, or don't find one with your missing photos, is it possible you had your Photos library on an external volume which is no longer accessible for some reason?

You have an uncommon problem based on the little information you have supplied. That may be why nobody has replied in the intervening two days.

Usually problems like this can be traced to something that happened coincident with an update rather than the update itself, but it is hard to guess what might have hit your library in particular.

Can you characterize which half of your pictures are gone?

For example, if pictures since a particular date are no longer visible, Photos may have switched to using an older library.

Are the counts of total photos and videos in your library reduced, or are specific items missing? If the latter but not the former, some filters may have become enabled that you were not expecting, or the ordering of items may have changed.

Any other information you can provide might help the various users know how to help you.

How did the pictures get into Photos? Did you import them from SD cards? Did they arrive through iCloud syncing? Did you transfer them from a phone using a cable? Or what? Is there a difference in the transfer technique from 2015?

And, of course, do you Time Machine or some other method to back up your files?