How to copy and paste as plain text on iOS?
In Mail on iOS 12, I use the "Share > Copy" trick:


Paste formatted
Select the just pasted text and in the context menu swipe right and tap on "Share..."
In the Share sheet, tap on "Copy". This copies as plain text!
Tap still selected text to get the context menu again
In the context menu tap on "Paste" to replace the selected, formatted text with its plain text representation


Still (too) many hoops to jump through but at least you don't need to switch between apps.
The "Share > Copy" trick might not be available in other apps, though.