All I said is that the deletions should be propagated. The presentation of this can only be ensured in locked-down systems, but the actual propagation is simple enough. Just as the server can send all users within a channel "so-and-so said this message", it can say "so-and-so deleted this message" — it can be left up to the client whether to actually remove it or perhaps just hide it, ready to be shown again with a click.

Obviously on Discord, one is tied to the official client's behaviour since the ToS discourages third party clients. However, for Slack, Mattermost, Matrix etc. clients, the possibilities widen up.

Ah, I read "deletion" as the actual effect, not the request for the action to be performed.