I just tried it on an iPad Pro running iOS 13 beta 7 and it works for me. Please note that you need to drag a repository from Working Copy (not from the Files app) into the sidebar of Textastic (where the "Local Files" and "iCloud" locations are shown).

The latest TestFlight beta of Textastic also has a new "Add External Folder…" option on iOS 13.

You can add iCloud Drive -> DEV as an external folder. Have a look at the manual at https://www.textasticapp.com/v8/manual/integration_other_apps/external_files_folders.html for details.

Yes, currently files are deleted immediately.

I'll try to find out whether it's possible for an app to move a file to recently deleted instead of deleting it immediately.

I hope to have this ready when iOS 13 gets released.

Textastic does restore tabs when the app is started  (for example if the app was closed by iOS or the device was rebooted). If you explicitly close the app through the multitasking switcher however, restoration doesn’t happen. This is the default iOS behavior.  

There were actually multiple bugs related to line endings because of a change I made.

This will be fixed in Textastic 8.0.3. Thanks again for reporting this!

Yes, if iCloud is visible in Textastic, then there should be a Textastic folder in iCloud Drive.

Textastic does not currently save files in "Recently Deleted" in the Files app - I'm not sure if this is actually possible.

But, if the file was uploaded to iCloud before it was deleted on the device, it should be recoverable on the icloud.com website.