Been with DP for 8 months. Most of her personal messages with friends (they're all in the tech field) are over Discord, with WhatsApp being what she uses for work and people she's not too familiar with. We don't use "normal" SMS because our messages are all multimedia heavy.
I've never had a problem with this. I was having dinner with a mate yesterday evening, however, and was told that Discord is primarily used by children and creeps. We're both 30. Would someone using Discord as their primary personal messaging app over WhatsApp/SMS be a red flag to you?