SUMMARY
The ability to disable thumbnail generation would be useful for some specific use cases of digiKam. It could also help save disc space and speed up the maintenance of a large image collection.

I use digiKam for reverse searching images in my collection of pixel art and almost all of these images are low resolution, have low color counts and load quickly due to their small file size and resolution. The thumbnail generator doesn't respect these low color counts when creating thumbnails, so in the majority of cases the thumbnails actually increase in file size and the thumbnail database becomes larger than the original files. I also have over 100k images in my collection, so generating thumbnails takes a really long time.

I don't really use digiKam for browsing my collection, I only use the reverse image search feature to find duplicates, so I don't really have a need for generating thumbnails.

ADDITIONAL INFORMATION
I actually managed to find a hacky workaround to disable them by denying write permission on thumbnails.digikam.db and the C:\Users\USERNAME\AppData\Local\cache\thumbnails folder (which gets used instead on Windows when thumbnails.digikam.db can't be written to, I don't know what folder is used for other operating systems). Now you'll get an error message when starting digiKam but the program still loads and instead of using the thumbnails database it just seems to load the files directly inside digiKam. Unlike when using a thumbnail database, it's slower, but in my use case it doesn't really matter, since the files are already small to begin with and load quickly.

While this hacky method works, it would be nice to have an officially supported way of doing this.