download of indesign and co. is not possible with my iOS Sierra 10.12.6

I'm afraid there is no workaround.  You MUST have macOS Mojave or later to use Creative Cloud.

See minimum system requirements below. Check each one because they vary by product title.
- https://helpx.adobe.com/creative-cloud/system-requirements.html

Adobe expects customers to have the latest operating systems  -- no more than 2 versions back  -- for security reasons.

Two more things

1. Sierra is the system that came with a late 2015 iMac. There have been many upgrades.

2. Don't look for iOS upgrades. iOS never ran on any Macs, it is for iPhone and iPad only. You run macOS.

According to this Apple web page you are wrong to say you cannot update your computer. https://support.apple.com/en-gb/HT210222

Of course you will need ALL NEW APPS, Catalina and Big Sur will not run apps older than 2020.

Direct Download Links https://prodesigntools.com/tag/ddl MAY help
-you MUST do what it says in Note: Very Important Instructions
-https://prodesigntools.com/adobe-cc-2015-direct-download-links.html#download-instructions
-this will only work until Adobe completely removes the old version(s) from the server
.
When you do have a download, be sure to copy it to at least one external device
-a usb flash drive or hard drive, or write to a DVD

i don't think there's any 2015 mac cannot be upgrade it's os.  it would have to be older than 2006 to have serious upgrade constraints.

upgrade your os.