P: I want to be able to exclude specific folders during synchronization

Yes: files already present in the desired folder locations can be imported in-place using the Add option. This offers you full selectivity on a file by file basis based on image previews, and also selectivity on the subfolders of the import source - meaning in the case of Add, just the source for some Catalog relevant operations. Besides reading the actual files in for preview generation, this can also include bulk keywording, the application of a chosen Develop preset / of a chosen metadata preset.

Besides of course Move or Copy or Copy as DNG when you do want that, with renaming perhaps.

Like you, I had an established folder structure before I ever used LrC. I worked out at that time what new auto-date-filing I wanted to happen with newly taken photos, guided by what LrC could directly deliver by itself, and put the Catalog in full charge of executing this. I had some slight reorganisation to complete for pre-existing photos till they conformed to this same scheme, but found that worthwhile to do. The benefit for me, is that I have since expended zero time or attention on file and folder operations - other than ensuring backup - for the files and folders that contribute to my LrC image library (and I started this in version 1). Not my concern any more: I've subcontracted the whole job out to LrC, it gives me a unified and predictable result back, so good riddance! The only residual intervention from me, as to folder management, has been choosing where my exports are to go.

Thank you for the suggestion. I'm aware of import's functions and abilities but this is about files and folders already present. My photographic collection predates the introduction of photoshop itself in 1987 and I have used many different programs and apps since then. The Adobe tools are by far the most flexible and capable out there in the prosumer space and I was pleased to find them. The very fact that LRC does not update as the file system does (unlike Bridge, for example) means that this suggestion has value; you can exclude a folder and it stays excluded. I would contend that this is a very useful tool.

You can expand the subfolders within an Import source and achieve selectivity on that basis. To be clear: the tool that's primarily intended AFAICT, and the one with the far richer settings and behaviour, is Import - not Sync Folder.

In my opinion, feature by feature comparisons of LrC against file and folder management utilities, are not useful. LrC does not even claim to directly report the live contents of a file system anyway - nor doubly so, to provide an environment that is well suited for bulk file or folder operations. The main thrust and focus of this product, is completely elsewhere.

I understood the feature you're requesting (you were very clear) but still don't understand the motivation. But my thoughts about this don't matter -- it's what Adobe and others who might vote for your idea think.

Thank you for your comment, but it's really very simple. I have some folders that I wish not to be catalogued within the entirety of a folder's contents. I do not wish to hold them separately, which is an alternative solution, for backup and security reasons. I don't think it needs any more explanation than that; you clearly grasped my suggestion immediately.

You absolutely aren't required to justify your request. But here's what Adobe says:

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https://community.adobe.com/t5/lightroom-classic-ideas/how-do-i-write-a-feature-request/idi-p/12386378 

How will the feature help your workflow?
Describe why you want the feature - include the problem you're currently hitting and how this new feature would solve that problem. Before working on a feature, the product team will need to understand how your idea fits in and may even come up with a better way of solving your problem.

Enlist support!
If you feel strongly about a feature, and you know other people do too, get them to vote!

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Your request will more likely get votes if people understand the motivation and how something similar might help them.

I don't think I have to justify a request. It's not a favour I'm asking, it's an enhancement to a product I am paying for.

You're more likely to persuade Adobe to implement a feature request if you provide detailed explanations of why you want it, e.g. an explanation of your workflow.