This is true. This is also true for for iOS and Mac OS developers and has been so for a long time. But it is certainly something that can be managed.
In AppleScript there are a lot of scripts written for a small group, a company, for a family, … For these it is almost always know what the minimum system level is before even starting on a script. For this situation if it runs on the oldest system then you’re good to go.
The main thing is that system versions are not as much of a problem unless you sell software to the public. There are a lot of scripters that create scripts to be used on a known minimum Mac configuration. When I wrote things for companies I always knew the minimum system configuration every time. So I literally ran functional tests on the oldest system level first and then finished my testing with various version levels.
I wrote scripts that interfaced with a lot of UNIX systems and other larger systems that produced output that the Mac worked with. The data configuration was always changing. So I just wrote a script that was easy to change. I added preferences when so the specification of the data being received could be specified and allow both old and new data could be accessed. The point I am making is that system levels and configurations are very manageable. Selling scripts on the open market is when it gets hard. If I’m selling to the public I can just developed something on the oldest system that will be used.
When it comes to Apple deprecating things that is something that all scripters would have to worry about. But again this can managed just like Apple developers do. It’s true you don’t know what will be deprecated but it is not like every year a large percentage of ASObj-C methods are deprecated. Right now I see a lot of methods and constants being deprecated and then brought back with a new name. All that is needed is to check the OS version and then use the old or new version depending on the OS version number. These particular types of changes leave the interface to the new and old way the same.
I’m not sure what your point is here as far as deprecations and minimum system configurations is. Is this a call for caution, a warning of danger or just some thoughts on something. I got the feeling it was a warning of danger but I I’m not sure.