I would look into containers and sandboxing. You can use tools like podman, bubblewrap and systemd to isolate software. I’m not sure what you are looking for exactly.

Firstly, you may also be interested in: https://containerssh.io/v0.5/

This is a similar software, but maintained. However, it doesn’t look like you limit networking with the Docker backend, beyond a simple on/off.

An even simpler solution, is to have the the ssh entry command not be the usual shell command (/bin/bash), but rather a command that starts a shell within a container. So something like:

podman run -it --rm -v "-v /HOST-DIR:/CONTAINER-DIR" docker.io/library/debian:bookworm bash would create a shell inside a short lived debian container (that is deleted upon disconnect) where a host directory is mounted inside the container.

As for mysecureshell, I would assume that since it is in the Ubuntu repos, it is still being maintained. But it’s possible, since it is unmaintained that there are unknown security vulnerabilities or other issues, but:

It’ll just be for half a dozen friends for when I want to give them larger files, or if I want them to send me full-resolution photos.

If it’s just for your friends, it may be okay to use a less secure solution if you trust them.

As an alternate solution: since you are looking for some sort of file searching, perhaps you could host an app explicitly designed for that, like Seafile or Nextcloud.

Have you looked at the mod_sftp for ProFTPd? It seems like ProFTPd is alive and well, despite its official site not supporting HTTPS! It seems like mod_sftp supports most of the SSH/SFTP-specific features, whereas ProFTPd may also offer additional features that complement the mod_sftp module. All in all, probably not a 100% match, and definitely not a drop-in replacement, but at least a seemingly viable/supported alternative?

Solution for what? What’s your use case?