Thanks @anontor but the problem wasn’t how to install a deb package but where to download it from. It’s not available at any of the links above. The instruction on their site, for Debian, consist on using snap. Following them (on a fresh workstation of course):
sudo apt-get install snapd
sudo snap install wickrme
installs wickme, but when trying to run it the following error occurs:
./WickrMe: error while loading shared libraries: libsqlcipher.so.0: cannot open shared object file: No such file or directory
anyway, the .deb files do get created using snap, so I later tried to follow the procedure in the wiki with them, but got the same apt-get breaking issue as @anonimac reported above.