> So, if activate.ps1 (and the other scripts for other shells) were updated to dynamically identify their own location rather than hardcoding the path, they’d be fully portable? Seems to be a fairly simple update.

I don't think the maintainers have gotten a request to do that. Personally I use a more complicated launcher because I need to set things like environment variables in my deployments, but a venv is almost an out-of-the-box relocatable Python environment. It's when you need to protect/compile your code that things get messy.

>> Since venvs on Linux symlink to the system’s Python executable

>Hopefully, the executable with which they were created, not the system executable except where they happen to be the same.

When you call /usr/bin/pythonx.y -m venv ./myenv, ./myenv/bin/python will link to /usr/bin/pythonx.y. Since there aren't many reasons to use a Python version not installed by the package manager, I call these the "system's Python executables", but sure, it can also point to /opt if you installed Python there.

They are in the sense that anything one would want to do with a venv, they can do with trivial effort. You don't need the activate script, you can just set PATH, PYTHONHOME and/or PYTHONPATH and be done with it.