My Ipod Classic had a button that you could slide across to prevent what you were listening to being changed. I cannot find a similar thing on my Ipod Touch

The sliding button on the top edge of iPod Classic was called the Hold Switch. As the manual says (you can get it from Apple - Support - Manuals) it was used to disable the iPod classic controls so that nothing happened if you pressed them accidentally. You could continue to listen to audio without the interruption caused by an accidental contact.

You can achieve the same thing on iPod Touch by pressing the sleep/wake button. I don't have one of these devices but the manual (here) shows that it is in the same position, on top of the iPod Touch. Similarly, it disables the touch screen while the iPod Touch plays audio and performs other functions.

The sleep/wake button is mentioned in the user guide for iPod Touch 7th generation using iOS 15 so it is still fitted to these devices.

You're very welcome. I have a 2005 iPod Photo with a colour screen which was later renamed as a 4th generation Classic. I use it many times a week and I still find it a fantastic product, just the right size too; I have a Nano also but because it is so compact I find it less comfortable to use. I use the Hold Switch all the time so knew what you were asking : )