So, I got my code and applied it on Catalina.
It seems they work via Family Sharing too if he ever gets a Mac newer than said: I mean it’s 199 USD for Logic Pro by itself, or if education you can get a bundle 199 USD for Logic Pro, Mainstage, Final Cut Pro, Compressor, and Motion.
I however have nothing to do with education at the moment. He is education staff, so if they do the random audit he’ll be fine. It says it can take 1-3 business days to email the codes it seems. I’m trying to contact my brother to buy the Pro apps bundle and send me the codes. It’s going to be a delay, but I’ll have the answer eventually. If purchasing on Catalina allows you to download the version that runs on Mojave that probably means it will work to download the High Sierra compatible version. I am pretty sure that the same version that runs on Mojave runs on High Sierra. I will be buying it on Catalina and downloading on Mojave though as I already have a Mojave partition for 32 bit apps. I won’t check on High Sierra as that’s going to take like an hour to install on a separate partition. I’m going to buy it anyway as I might as well make my current Mac Mini still useful, and it’s one of reasons I’m getting a new M1 Mac. I’m about to buy Logic Pro and I’m wondering the same about running it on Mojave.