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Not necessary anymore, you can message yourself in WhatsApp now. They even sent me a notification in that regard. (That is recent though and I wish I'd known that (clever) workaround before this was possible)

Recent? Unless you mean the notification, sending yourself a message has been possible for a long time. Just to make sure I know what I'm talking about I checked the conversation history with myself and the earliest messages are from 2016.

Signal has a "notes to self" chat that I use to send smallish files. Anything large though goes via syncthing.

There are many memes on reddit that people use offical Ronaldo/TaylorSwift/CocaCola/whatever WA accounts for this.

iMessage or the notes app works with mac and iOS or a combo of those.

iCloud Drive probably does too, but it’s horrible in my opinion.