Hello,
I can see that this suggestion has already been made in 2017 and 2018.
But I believe it's worth putting this suggestion back on the table, as Signal App has seen an expentional growth since a few weeks, as WhatsApp aims to force users to share data with Facebook to keep using their messaging service.
I think adding Signal now would be a very relevant, useful and clever move from Opera. Making Opera always smarter, effective, pragmatic and ahead of the competition 
Thank you very much and congrats for your good work !