Okay, I was wrong. The [bionic repository] only includes packages for bitmask-vpn, not riseup-vpn. The [buster repository] does include riseup-vpn, so that's what you need. Note however, that there is only a 64-bit binary package for riseup-vpn. There is [no 32-bit package], so if you have a 32-bit system you're out of luck, unless you compile the program yourself.
: http://deb.leap.se/client/dists/release/bionic/binary-amd64/Packages : http://deb.leap.se/client/dists/release/buster/binary-amd64/Packages : http://deb.leap.se/client/dists/release/buster/binary-i386/Packages
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