If you’re already basically competent with the OS you’re using and have a machine with a decent connection that can be used for mail then it actually isn’t hard.
I cannot claim prescience, because I started ont the trade when sendmail was THE mail program (though ed was no longer THE text editor): but consider, had I listened to the sendmail bashing crowd a couple years later I would have gone qmail (!!!). For not having done that, I thank the almighty weekly.
That would work if mail providers actually used Sieve, but almost none of them do. I've never understood why.
Can't tell if you're serious or not - but if you are, just looking at man sieve is enough to understand why no sane person would give an average end user access to that. I mean I used it for many years but I'm a nerd weirdo.
You don't have to force an average user to hand edit Sieve files. You can put the same kind of GUI or web interface on top of a Sieve file that current mail providers put on top of whatever hand-built custom non-standard filtering rules system they currently have. To the end user it would look the same.
But for an end user like me, who already has a huge number of filter rules expressed in Sieve and would like to be able to upload them to an email provider, not supporting the Internet standard format for filter rules is a showstopper. I don't want to have to re-input all my filter rules into an email provider's web interface one by one. I want them to be able to understand the rules I already have in the Internet standard format I already have them.
From a quick google, there does seem to be a Roundcube plugin for managing Sieve filters, but man yet another dependency (or two, as it seems you need a separate daemon as well) in an already brittle (by the time you get to this point) stack...
Look, I understand where you're coming from, and I used Sieve for many years, but it's 2019 - something is not an 'Internet standard format' when the number of users actually using it can be expressed with 4 or 5 digits at best... This horse is not just dead, we're snorting its ground up bones by now. Nobody caters to power users any more. Back when power users made up a sizeable portion of the internet population, yes, but not in 2019...
Well, there is no other standard format at all, so basically you're saying we might as well just give up trying to have an internet standard for email filtering. That doesn't seem like a good outcome to me.
Even if I'm way off base about the mail server set up being this way, some people still consider this type of "saturday afternoon" project hard compared to something that just works out of the box without any fiddling and can be set up in 20 minutes. I'm not trying to be elitist by suggesting these people are dumb or lazy, just that the semantics of what "hard" is varies for people, especially with little projects that require fiddling/tweaking/looking for outside help.