Have the ability to specify illegal character replacements.

Have the ability to specify a certain default character to be used for illegal character replacements / ? < > \ : * | "

I typically prefer a simple _underscore_
This is also handy when you have used another ftp program which has already done that on numerous files.

It is also important to retain the current option of displaying the % and ASCII character code. This is a nice feature.

This is actually (very) old behavior of WinSCP.

You can revert to it using:
See https://winscp.net/eng/docs/rawtransfersettings
(edited in 2022)

Though it gets lost anytime you submit the Transfer settings dialog.
https://winscp.net/eng/docs/ui_transfer_custom

I see. I was just going through the GUI menu and prefer to work that way. But if it supports it via command-line, ok.