The “x” that closes a single safari tab or window on my iPad has disappeared

Perhaps something has malfunctioned - unusual, but possible.

I suggest force-closing the offending Safari instance...

• With the offending Safari window open, swipe-upwards from the bottom edge of the screen, just enough to reveal the dock.
• Tap the Safari icon on the dock to open the Exposé view (you’ll remember this from the original linked document) of your open Safari instances.
• Swipe the window corresponding with the offending instance off the top of the screen.
• Tap on any free space to finish.

I don’t quite understand the issue that you are attempting to describe - but this link may provide some greater insight into Safari and its handling of spaces (instances) and tabs:

https://discussions.apple.com/docs/DOC-250002077

If this doesn’t help you to resolve the problem, perhaps it will provide material with which to further explore and describe the issue that you have.

I hope this proves to be helpful.

The reference served its purpose - in that you can now better explain your perceived problem...

I believe both you and I now better understand your question.

What you likely see is the iPadOS multi-tasking environment doing its job - its function being somewhat different to earlier versions of iOS on the iPad. In past iterations, you were able to close your Safari window simply by closing the last remaining tab/page. This is no longer the case.

• If you have multiple tabs open in a single Safari instance, the currently active tab will have the “X” button to close the tab.

• Should you have a Safari instance with only a single open page (not multiple tabs/pages), then the “X” close button is not present.

In context of the system and UI design, where closing an App is not power-efficient, this makes sense - as you simply switch to a different task when needed.

The iPadOS system optimisation transitions tasks to an inactive state whilst you are using a different App - saving the App state to non-volatile [slow] flash memory, for immediate recall when later returning to the App. Whilst inactive, RAM is released to the operating system for use by other system processes.

Closing and relaunching an App consumes considerably more power - as loading and initialising a process from scratch is significantly more CPU intensive than reloading the [already initialised] saved state.

I hope this explanation and insight provides some clarity as to the behaviour that you see. Your explanation indicates that you iPad is operating entirely as expected for iPadOS13.x.