I mean, technically they asked for a SPAM filter, but in reality, when they’re subscribed to emails and lists they’ve signed up for, they’re marketing emails. Spam would be unsolicited and usually come from their data being sold off. The very fact those notification emails or marketing have the "unsubscribe"button lowers their spam score so they hit the inbox.

So, Proton, for example, claims to not sell off or monetize your data based on strict privacy laws from Switzerland. You don’t have to believe their claims, fine. They’re derived from scientists though, not businessmen. Didn’t seem like they’re prioritizing big money. So keeping spam away, this is a good way to do it. Doesn’t mean all the other companies OP has subscription emails for hasn’t sold their info, so it won’t be a fix-all.

OP sounds like they need to go through their emails occasionally to just unsubscribe to help clean it up lol. Really, using rules to filter key phrases would be easiest. The reason the aliases are suggested to help, bc the emails you really want to prioritize from any friends, family, or services you want to focus on, use the primary address. All others, like shopping for insurance, retail accounts, etc, use a junk one so you’re not bombarded. You’ll get a ton of marketing regardless, so that’s a great way to cut out the “spam” notifications.

I would go one step further and give banks/credit cards their own alias too, to avoid reusing the same address to help cut back on data breach info. I exclusively have a login email address and an alias for everything else. That way no one will ever know my login address to get to my account, unless the hosting company themselves are breached.