Re-Installing Windows 7 on mac
You do not have to downgrade to a earlier version of MacOS, you just need to install High Sierra in a new APFS volume in the same APFS container that Catalina is using. You can download High Sierra by accessing the link on the Apple website How to upgrade to macOS High Sierra. After High Sierra finishes downloading, you will get the following popup message. You can click on Quit to exit the installer.



To install High Sierra, you will have to copy the installation files to a flash drive. See the Apple website How to create a bootable installer for macOS for instructions. Once booted from the High Sierra flash drive, you can install High Sierra in a new APFS volume.


  Note: The installation of High Sierra on my Mac consumed about 29 GB of space. If you are space limited, then you may have to temporarily move some of your personal files off of the installation drive.


After finishing installing High Sierra, you can use the Boot Camp Assistant included with High Sierra to install Windows 7. 


  Note: Apple documentations does state that the Windows 7 ISO should include Service Pack 1 (SP1). You can download a Windows 7 ISO with SP1 from the Microsoft website Download Windows 7 Disc Images (ISO Files).


During use of the Boot Camp Assistant, you will encounter the following window. 



If you click on Open Website, you will be directed to the website Install Windows 7 on your Mac using Boot Camp. You should follow the directions given on the website.

After Windows 7 is installed, you can boot to Catalina and use the Disk Utility application to remove the High Sierra volume.

Some Comments about 2015 and Newer Macs

Apple and Microsoft intended for Windows 7 to BIOS boot on Macs. The instructions given in this answer should install Windows 7 to BIOS boot. However, starting in 2015, Apple starting eliminating the ability for Macs to BIOS boot. This is why the links to download on the website Install Windows 7 on your Mac using Boot Camp end in 2014. In your case, you can either try installing Windows 7 without the Boot Camp Support Software (Window Support Software) or try the Windows Support Software that can be downloaded by using the Boot Camp Assistant.

A Note about Windows 10

If you have a Windows 7 product key, then you may be able to use this product key to install and activate Windows 10 for free. When asked for the Windows 10 product key, just substitute your Windows 7 product key. The latest Windows 10 ISO can be downloaded from the Microsoft website Download Windows 10 Disc Image (ISO File).