The first release has not been published yet. Add it once and apt keeps Outreach up to date:
curl -fsSL https://apt.guahva.com/outreach.gpg | sudo gpg --batch --yes --dearmor -o /usr/share/keyrings/outreach.gpg && echo "deb [signed-by=/usr/share/keyrings/outreach.gpg] https://apt.guahva.com stable main" | sudo tee /etc/apt/sources.list.d/outreach.list && sudo apt update && sudo apt install outreach