The reason they don’t support Monero was adressed in a recent podcast “OptOut” with the CEO of Proton.
TLDR; they fear the biggest companies for auditing their finances, won’t work with them if they do. Which is reasonable, they are a business after all.
I have been following their advice for years. I have replaced a couple of services with the suggested alternatives, and use GrapheneOS.
It is also useful to check their forum for discussion on why something is added/removed/not added
If you are new to the whole “privacy-thing” my advice would be to start figuring out your own threat model first. They have some guides for that.
Also remember that even though something does not make the cut to be mentioned as a recommendation, it might still be an improvement over what you currently use. The forums are great for discussions regarding that.
And it can be sorted alphabetically in all software. That’s a pretty big advantage when handling files on a computer