P: Colored "pixel" artifacts/subdued blacks in shadow areas (older Macs) (also visible on CR)

I came to this experimentally. Maybe it depends on the computer configuration. LR version 12 will not read the catalog from version 13. It works for now.

After exporting about 1000 photos, I did not notice the artifacts.

Do your exports render correctly?  There are some comments in this report that 13 has a bug and GPU is used for exports even if it is disabled in the settings.

correction:

1. Uninstall Lightroom

My temporary solution for Mac Pro 2013:
1. Remember Lightroom
2. Install v. 13.0
3. Use of a graphic processor = OFF

Thanks to this, I could use the catalog in the latest version.

Attention.

If you have to go back to version 12.xxx - write all changes to XMP files before. Because you will have to create a catalog again, but you will not lose work on processing.

However, Adobe support still advises not to run Creative Cloud on Sonoma / OS14. What does this mean? I should downgrade the M3 computer to OS 13?

Not sure why they would say that. Lightroom Classic runs just fine on Sonoma and as far as I know there are no known incompatibilities with anything in creative cloud. You really do not want to downgrade an M3 machine as already mentioned. 

"Some people have found that disabling GPU support also works, but apparently export ignores this setting and still uses GPU and thus can remain impaired."

Specifically, due to a bug Export in LR 13 ignores the setting of Preferences > Performance > Use Graphics Processor and always uses the GPU. But LR 12 obeys the setting, so disabling the graphics processor in LR 12.5 will fix both the display and export problems with artifacts.  LR 12.3 is the last version in which people with this problem can enable the GPU.

even if my question about the downgrade was meant ironically - many thanks for the useful hint, Malcom!