Why has ‘Forget This Network’ disappeared?

You can still remove WiFi networks from iPad settings - whether or not you are near active WiFi networks:

Settings > WiFi - tap the blue Edit button at top-right of the page

With your WiFi network list exposed, tap the blue "i" icon adjacent to the WiFi network that you wish to edit settings - or delete. To delete, select Forget this Network

if it's still on and broadcasting it's SSID then it's the default behavior to show up on the list of available networks, that is how it have always been same thing with computers and the likes

See my earlier reply.

The router doesn't need to be turned-on - or even exist - to be able to remove it from your iPad's settings.

You're very welcome. Sometimes it just takes a little insight from a regular user of iPadOS 🙂