I have no idea what would happen if I, for example, downloaded V 3.0.1 of Onyx, and tried to use it, given that I am now running El Capitan. I don't want to try!
Whenever I have upgraded to a major OS (e.g. from Yosemite to El Cap), the old version of OnyX has refused to open, saying that it is not compatible. I assume that the same would happen if you downloaded an older version.
Yes, that makes sense. As it is, before I upgrade from one OS to the next one, I already know that at least 5 of my critical applications need to be compatible with the new OS, or else I will wait for upgrades to such software. That is exactly what happened prior to El Capitan coming out. 1Password, Office 2011, SuperDuper!, and Onyx had updates for El Capitan compatibility prior to the release of OS 10.11, and for TechTool Pro, it was shortly after OS 10.11 was released. It was a couple of weeks later that I upgraded from OS 10.10.5 to OS 10.11.1.
I have read of some "rare" instances of software that needed updates within releases of an OS. Fortunately, I have never needed to do that. But, it could be something to be aware of. In fact, I can't remember any such need to "mainstream" third party apps like 1Password, SuperDuper!, Carbon Copy Cloner, Onyx, etc. The recent one for TechTool Pro was somewhat rare, although as MacManiac previously pointed out:
"As I recall, MicroMat was burned pretty badly several years ago when one of the functions in an earlier version of TechTool Pro (that had worked in testing against the pre-release beta of the OS) played havoc with the final release due to some last-minute adjustments that Apple made to the final version...."
I certainly sensed that while reading the TechTool discussion regarding El Capitan compatibility of TechTool Pro. Prior to the official release of V8.0.3, the Micromat participant in the discussion mentioned quite a few times about insuring TechTool Pro would be compatible with OS 10.11. I guess I took a little risk by moving to OS 10.11.1 after V8.0.3 of TechTool Pro came out, even though it was "only" certified" with OS 10.11 (same was true, of course, with the other apps I use). And, in fact, and knock on wood, things have been fine for 10.11.2 and 10.11.3. 10.11.4 has been in beta testing for quite a while, and I have not seen anything regarding third party product compatibility and OS 10.11.4.
Of course, I insure to apply each update for any of my apps that come out right away, so that I am current.
