Can I save backlog items or tasks as templates and reuse them?

What you can do is just create normal backlog items and copy those when you need to reuse them.

For example, create a backlog list named "Templates", and place the template backlog items there. Then copy those backlog items when you need to reuse them somewhere.

For tasks, create one or more backlog items to hold the template tasks, and then copy the tasks from there when you need them.

If you use templates like this, you probably want to turn off the setting that adds "Copy of" to the name of the copy, so that you don't have to remove this yourself each time you copy from a template.

If you would like to use the same template backlog items in multiple projects, you can instead create a dedicated project for templates, and then copy the template backlog items from there, between the projects.