This should also work in iOS 13 if you installed a monospaced font using an app like AnyFont.

On iPad you can hide these using the "Additional Keys" setting in Textastic. 

I can reproduce the problem with Japanese text. I'll try to find out what is going on.

I could reproduce and find a workaround for this problem just yesterday. It will be fixed in the next update.

This seems to be a JSX syntax definition for TextMate 2 which injects itself into the built-in JavaScript syntax.

Textastic only supports TextMate 1 (and Sublime Text 2) syntax definitions, which do not support injection.

Currently, you can only use copy/paste to copy text from a code editor tab and paste it into to a terminal tab.

Textastic only supports syntax definitions and some preferences found in Sublime Packages. It does not run Python code from Sublime Packages.

I just tried to reproduce this, but it works on my iPad.

Did you maybe change the keyboard shortcut in Settings -> Keyboard Shortcuts -> File -> Upload This File? If yes, you can reset it there.

What does the overlay show when you press and hold the Cmd key on your external keyboard?

You should already see Markdown headers in the symbol list. Does this not work for you?

Yes, I know what you mean. I have already an idea for how to implement this, but would like to know what you would expect:

Should this always create a plain text file with UTF-8 encoding or should this be configurable? (for example, maybe you want a Markdown or HTML file instead).