Really? Only the very unstable Yosemite and ElCapitan. Why in heavens name???
Apple is frequently changing stuff in its OS versions, in a non-compatible way. It’s not possible to check an application against too many OS versions.
Non officially, you could run WaveLab under 10.9, however.
I suppose it’s relative to what other (older?) software applications you might use but I find 10.11 very stable. 10.10 had some minor issues on my system but nothing serious that made me want to go backwards.
WL9 will run on 10.9 but not officially supported. Maybe you’ll have to decide the best for your workflow:
10.9 (or perhaps 10.8 with some luck) and unsupported WaveLab
10.10 or 10.11 and supported WaveLab.
There’s really nothing about 10.11 that is unstable here and I have WaveLab, Pro Tools, and lots of 3rd party plugins. All is good.
As you say: There is a question of older software and a lot of money and installation time involved. WL9 does not install on 10.8. That is how I found out. I did not check before I bought the upgrade as I found it unlikely it did not have 10.8 support as the DAW it synchronizes with, CB8.5, is 10.8 compatible. I use 10.11 on my laptop for other businesses but I will not upgrade my studio (a MAC Pro) in hopefully a very long time. So I guess I have a WL9 upgrade for sale.
Alas only 10.11 is available from Apple unless you previously have downloaded 10.9. Same goes for 10.10. I have none of them in my Apple store. Great policy from Apple.