What cloud storage provider officially supports this tool or the protocol used by it?
The instructions look mostly like a reverse engineering effort that can fail anytime for any reason. Of course you'll get some notification, but still seems a thing to avoid in principle.
What if we looked at all of the cloud providers APIs, which may or may not be similar to one another, may be in constant flux, and may have undocumented quirks, and hid all that complexity inside of one black box that provided a single clear API. The guts of that black box might be kinda ugly, and might need at lot of constant updating, but for a lot of users (including me), that is more appealing dealing with all the raw complexity of the various cloud providers "offical" APIs.
There is no rsync protocol, only whatever protocol exist already. Most providers will have their version of S3, so if a provider wants to implement a strict copy of the AWS api they will be compatible. They can also provide an SFTP API and rclone will work.
It's a wrapper around a bunch of API's. If that makes you nervous then wait until you hear about how the rest of the modern web is strung together. It's been working reliably for me for a decade.
That's wonderful! Full endorsement of the support of rsync.net.
I sincerely doubt most tools will be supported by most providers (by usage), however, and I question the evaluation of tools by this metric. Support is generally a fairly bad metric by which to evaluate tooling—generally speaking, if you care which client is consuming an API someone has done something horrifically wrong.
The instructions look mostly like a reverse engineering effort that can fail anytime for any reason. Of course you'll get some notification, but still seems a thing to avoid in principle.