Why old filters like Vivid not available in iPhone 16?

I presume you mean in the Photos app? If so, the Vivid filter is still on my iPhone 16, though it's a Pro model so perhaps things are different. In Photos, tap on a photo. Look to the bottom of the screen and tap the little "sliders" icon. At the very bottom you should see "Filters". Tap that and you'll get a slider set of photos with various filters, one of which is Vivid; at least it's there on my phone.

Regards.