The ability to make text invisible or 'replacing' it with '___' using the Character Style
I recently worked with a publication that needed to hide part of the text in different places. The full text also had to be shown next to it.
It would be possible to arrange this by means of GREP-styles, if it were possible to use some kind of pseudo font with zero character width or something like.
Also it would be useful having an opportunity visually replace any symbol with underline (such an idea is often using in a language textbooks) using character styles.

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Tor Stokkan commented
Make character color transparent.
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Viacheslav Horobchenko commented
Set the font color to be the same as the background (paper) color and the underscore color to black or whatever you want. This can be styled as a Сharacter Style
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Filip Blazek commented
Hi, this could be solved very easily with a font. Create a font and replace or characters by an underscore. (Nevertheless, copy-paste to another app would reveal the hidden text.)