There are usually 2 versions supported concurrently and one in development which you can download from the github repository:

New major versions are generally released at least once a year (more or less - when it is ready), followed by minor updates as needed.
You can find some statistics on an older version of the project here: openhub: xpra.

Versions older than 5.0 are no longer supported.
Some platforms have more restricted support.
The html5 client is compatible with all currently supported versions.

If your distributor ships an older version or an EOL version, you should complain to them for not updating a package with known bugs and serious vulnerabilities - more information here: Distribution Packages

You can mix versions between the clients and servers, as long as they are reasonably up to date.
Versions 5.x are compatible with all versions since v3.1 (except for Python 2 builds of v3.1.x).
Version 6 drops compatibility with versions older than 5.

You can find binaries for tagged versions for the supported platforms here: https://xpra.org/dists/

You may find beta builds of any branch or trunk here: https://xpra.org/beta/

More download information here: downloads