Founder of Svix here.
I don't know if I'd call us SaaS first, though we definitely spend a lot of time, money and attention making sure our SaaS offering is as reliable as it gets if that's what you mean.
FWIW, I've been an OSS developer and maintainer for all my life (my previous business was also open source), and open sourcing Svix was always the plan. It was literally one of the first tickets I opened for the team, and our first public commit was in Feb 2021.
We've indeed recently announced the open sourcing of our webhooks dispatcher[0]. It's written in Rust, so also a single binary with no third party dependencies. We do NOT however offer in memory storage. This is on purpose, as persistence is a requirement for robustness.