First, ASCII support is good enough for stopping using digraphs and trigraphs. You do not need UTF-8.

Second, this link might be what you want to read:

On Sat, 16 Jan 2021 at 03:35, corentin.musard--- via Std-Discussion <> wrote:
There were useful long time ago, but with the use of utf8 everywhere why not deprecate or even remove them?                                                               
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Yongwei Wu