Odin 2 (audio) soft synthesizer for macOS that was built using newer JUCE framework that should run as far as in 10.9.
You probably need a MIDI controller to play on it. This is an independent application, not a plug-in. It sounds pretty cool.
We are not great fans of macOS but we sometimes need to also use one macOS version (that machine would waste in other case) to run things, considering it as a Unix (Linux-like) environment and we attempt to pick what frees things up for you.
Update info: you can use Virtual MIDI Piano Keyboard or MidiKeys to play using keyboard

Don't be too shy, feel free to give it a try, hesitation isn't good for you when it comes to that type of goodies, trust me, I'm your best advisor :-), there should be no harm in that unless you are a hermit or an orthodox Jew, in which case you probably wouldn't need computer either lol. I believe it's quite easy to download it and to do something with that (well, perhaps relatively, it may depend on the item), after all what have you got to lose? Don't take it incorrectly, I'm simply incenting you to have a go, otherwise you could i.e. wait for unspecified amount of time not knowing and it's good to evaluate it potentially, I don't know why people like something and don't use it BTW or they like it and suddenly they don't like it without a cause, that's peculiar, it's a soft promotion/making that more apparent to ya in order to incent you to take advantage of using it actually yourself, I hope it's not too over you.

DISCLAIMER (it's just a precaution and because of mutual safety I guess)
This software/resource is provided "as is" without warranty of any kind, either expressed or implied and such software is to be used at your own risk. While it was created taking many aspects and factors in mind, the use of this software/resource is done at your own discretion and risk and with agreement that you will be solely responsible for any damage to your computer system or loss of data that results from such activities. No advice or information, whether oral or written, obtained by you from us shall create any warranty for the software/resource. Additionally, we make no warranty that: (a) This software/resource will meet your requirements, (b) This software/resource will be uninterrupted, timely, secure or error-free, (c) The results from the use of this software/resource will be effective, accurate or reliable, (d) The quality of this software/resource will meet your expectations, (e) If errors or problems occur in connection with a download of this software/resource obtained from the links on this website, they will be corrected.