I've tried reaching Microsoft by phone, email, automated tools for email administrators, and their web forms for requesting whitelisting in email deliverability. Microsoft didn't provide any rational why they are filtering email from small email providers like this, nor did they provide any opportunity to mitigate their filtering.

Probably it's more convenient for them to trust X big players rather than checking one by one. And somehow an empty spam folder looks better than a full one, as it gives the user the feeling that the service itself doesn't receive spam or does a very good job at preventing it.

That's just the thing: "doing a good job preventing spam" means throwing a lot of stuff in the spam folder. I think everyone would agree that silently dropping emails is bad, even if you're really sure it's spam.

I agree, it's just that you're not the average user. I am also surprised to see the spam folder surprisingly empty all the time, almost as if my email addresses were unknown to the world (which I am pretty sure they are not).

The average user probably sees this full spam folder as something he/she is doing bad, or the service itself is bad.

It's not just google or microsoft. I think most email providers do this, I might be wrong though.

You can create anonymous servers on DO with go-betweens like Bithost, allowing you to pay with cryptocurrency. This is a pretty easy solution for people with bad intentions (bots, scraping, black hat ...)

I run my own VPN on DO and almost always get hit by ReCaptcha. Even Google.com will put up a "we've noticed some unusual traffic from" page. Some sites like Zalando completely block me with just a plaintext error page.