Going to Rclone downloads I see "Solaris" icon, but it is not selectable for download.
Going to "Older Downloads" section I navigate to https://downloads.rclone.org/ and I can download a ZIP for Solaris just fine. So it is available and it Works.
Please, do correctly link "solaris" icon in the download page to the right binary.
I'm just letting you know the entire page has the wrong links as it's pointing to 1.59 which doesn't exist as it should be 1.58 and we'll get it fixed.
Whatever was there for 1.57 wouldn't matter as this point as it's already been replaced so it's irrelevant.
We just have to fix the links for 1.58 for the whole page.