Create bootable mavericks installer on Catalina?

Thanks for your reply.

A lot of this is typical Apple nonsense. For instance, they claim that I can boot in "Internet Recovery" mode, which will install a version of the ORIGINAL OS supplied with this computer (which was OS10.9.1, Mavericks release 1). Three times, that was a non-starter...got to the point where "Apple" wanted to check my "machine" for "compatibility"...asked which drive I wanted to install Mavericks on...and then did not show any available drives (as, no doubt, it considers an APFS drive incompatible with a Mavericks install).

Luckily for me, after a decent night's sleep, I came up with the solution. I simply plugged in an old backup of the iMac (kind of current, but a couple months old) to the USB port and, restarted with Option, and voila, I was given a startup disc choice.

Step 2 was to take out the old HDD backup from the encolsure and replace it with the salvaged SSD from the iMac. Bingo cheesecake, the MBP is running 10.9.5, and looks for all intents and purposes like my old iMac.

Now, to get rid of Catalina and its nervous nelly parenting style of "No, you can't do that with your computer" crap. And install the OS that I actually want to use.

Thanks for helping me here.

Yes, the internal SSD in the MBP is the Mac original. And the "boot" SSD from the disabled iMac was also a compatible replacement for the original Fusion drive in that computer. I had that replaced when it failed back in 2020.

The moral of the story here is:

1) don't give up, just put it down for a while and rest

2) ALWAYS backup, using a utility that makes a "blessed" copy of the drive (which means it will start up the system - two brands I have used are Carbon Copy Cloner, and SuperDuper). If I hadn't backed up the iMac, I would have never been able to try starting from that backup. When that worked, the next step was to boot from the salvaged iMac SDD (via a USB3 enclosure). That started up quite quickly, and I was able to clone the external startup disc to the MBP's internal drive using the aforementioned SuperDuper.

Also, finally, shoutout to the Internet Archive, for hosting downloadable copies of ALL Mac vintage OS.

Too bad the geniuses at One Infinitely Dumb Loop couldn't figure out how to do that. There's reasons for staying upgraded, and reasons for not upgrading. Someone there should learn that.