It's been more than two months since Catalina was publicly released. This happens EVERY YEAR with new OS releases and hardware. If anyone is surprised by this then they have not been paying adequate attention. PLAN AHEAD. Order stock in advance if you anticipate not being able to support the next release of macOS.
I continue to be amazed by the fact that IT admins don't manage their deployments and inventory. This is all within your power to accommodate.
This has been the modus operandi of Apple forever. Anytime new hardware is released after a new operating system, that new hardware can only run that current operating system and newer. Just do a search on Jamf Nation for any of Daper @donmontalvo 's posts with "forked by Apple" in them. You'll see that we have this same discussion every year.
Late to the discussion here, but I echo what's being said above. This is SOP for Apple for a long while now. It doesn't always happen with small speed bumps to the same model, but whenever they introduce a more significant hardware revision or an entirely new model, like in this case, they ALWAYS tie it to the latest OS release and it cannot be downgraded.
In all seriousness, no-one should be surprised by this at this point. In fact, I would have been very surprised if the 16" MBP could have been downgraded to Mojave.
That may be your problem. I believe Jamf Pro 10.15.0 was the first release officially Catalina compatible. If you do SSL inspection on your network be aware there are additional servers that require whitelisting when moving from Mojave to Catalina for initiating DEP enrollment. (humb.apple.com being one we hadn't previously needed whitelisted) .
A comment I saw above stated that because new hardware can't have the software downgraded they will never buy an Apple product. Ultimately, that's your choice, but the truth is that Apple does this for a myriad of reasons. They tune their hardware to work most effectively with the most recent release of the software. They would have to do a ton more work to do to also prep Mojave for whole new piece of hardware with new a chip layout.
While Windows tends to not care and allow you install it on just about any piece of hardware, that's not true of the companies that make hardware drivers for their hardware. Windows doesn't build their own drivers for hardware because there are so many iterations of Trackpads, Mice, Keyboards, Wireless Cards, SSDs and HDs, RAM, and everything else that they let the hardware manufacturers build their own software drivers. And when new hardware is released from those companies, they don't generally support backwards compatibility to earlier versions of Windows.
If you don't like it, don't buy it (which was stated quite clearly)- but don't pretend that other companies aren't doing the same thing.
It is this kind of attention to their hardware and software that brings me back to Apple over and over. There might be machines out there specced the same or better with the same build quality as Apple, but it won't have the same level of software parity or quality (not to say that Apple is perfect, it's not - it's just better).
@allanp81 As a counterpoint, I am soooo glad that when I'm installing macOS on a Mac I don't have to worry about bundling the correct drivers to install because the components were likely to differ from machine to machine, even ones purchased in the same order. Needing the macOS version that shipped on the machine, or newer, is a much simpler requirement, and one I'll pick any day.
For whats its worth, as others have stated it has always been this way, deal with it., thats no reason to not buy Apple lol. Also, I find is a lot easier to deal with the new OS on Apple forced on us than all the various old versions of Windows that seem to stick around forever. I'll take the Apple way 8 days a week.