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I read that with Image ⇾ Mode ⇾ Indexed  I can get the whole image black and white without grey, but how to only apply this to a selection, other than copy the selection out – apply – paste back?

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    Hi. Welcome to GDSE. Unfortunately it's not possible using Indexed colour. You can't use it on a selection only. Indexed colour is a colour mode, not just an effect. Colour modes apply to the whole document. Try using Colours > Threshold instead, or maybe a combination of Desaturate, then Threshold.
    – Billy Kerr
    Apr 18 at 16:00
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Just make a selection and use the Threshold tool on it:

enter image description here

Note that when doing so you have no fine control on which colors become black and which become white, so a more complete solution is to convert to monochrome first (Colors ➤ Desaturate ➤ Desaturate or Colors ➤ Desaturate ➤ Mono mixer) before applying the threshold.

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You can do this with the G'MIC Filters that come as a plugin for Gimp. These filters are applied to a selection if there is one. Below is using the filter Black & White > Stamp:

enter image description here

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