If you have a folder shared with 10 people, most likely only a few files will be accessed by others and the rest is dormant on all but one machine. Downloading and storing all these files is an expense in transfer fees and to some extent a waste of local disk space.
For that reason, cloud sync tools no longer copy everything up front, but transfer on-demand. Most tools have an option where you can choose "Make available offline" that will make a specific folder always synced.
That said, silently excluding a folder is very problematic, even if there is a good reason for it.
I work on the open-source Duplicati backup tool (https://github.com/duplicati/duplicati) and we take special care to not silently skip things as this is likely to cause problems when you want to restore later. For instance, you will get a lot of warnings if you try to make a backup of a cloud-synced folder, as the cloud-sync cannot keep up with the speed of the backup.
If you like the pricing of B2 but not the backup tool, you can use a B2 bucket (pay per usage, not flat rate) and have Duplicati back up to the bucket.