shkspr.mobi
Signal's new(ish) URI Scheme
The Signal messaging app launched signal.me URls for this purpose back in 2021. It allows you to share a link and have your signal client open up a chat with that person. Then, towards the end of 2022 they added support for their own scheme! sgnl (although it doesn't appear to have been submitted as an IETF draft, and isn't listed on IANA).