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WhatsApp seems set on introducing ads, but they won't appear in your inbox
(www.androidpolice.com)
This is a most excellent place for technology news and articles.
Should work pretty well even, since they use the same encryption protocol.
Won't happen though. Signal is very much against interoperability, and they're not big enough to be considered a gatekeeper that will be forced by law.
Fun fact I learned not long ago:
Even iMessage technically wouldn't count as a gatekeeper, since it is in fact below the usercount threshold the EU defined.