I'm running the latest version of Animate and working for a client with an older version. They can't open my .flas and there appears to be no way to save down a version. Has Adobe removed this very vital functionality?
What Adobe did was far more useful. From CS5.5 onwards the file format didn't change. Features change, but the file itself didn't (not counting the custom FLA types there are). You can open an AS3 Animate 2017.2 FLA in any version back to CS5.5.
If they are using CS5 you would need to install CS6 (which you can do from the Creative Cloud menu) and do a Save As to CS5, and then send that file.
If they are using CS4 you would have to track down CS5.5 or CS5, and do another Save As. In that particular case your best go-between would be CS5.5, which can open the Animate file and Save As to CS4.
The general case was always that there was support to save back two versions. With the change of approach you can save back 12 versions.