In November 2017, I looked again at my inbox. According to my Gmail Meter measurement then, after four months I received 261 emails instead of 635: 59 percent fewer. That was my situation at that time. It is not a result others can expect from the same steps, but it shows how many low-value messages can enter an inbox.
Start with notifications
I disabled email notifications from services when I did not need them. Not every message can or should disappear: security alerts, invoices and client mail need different treatment from a notice about every small event. Check what each sender’s message does. If you already inspect a service at set times, it may not need to email you for every change.
I also unsubscribed from lists I no longer read. That does not mean newsletters are useless. A good mailing can be valuable; the question is whether that particular mailing helps you now. Keep what you really want to receive.
Filters without losing important mail
For some recurring messages, I used Gmail filters. A filter can label or archive automatically, keeping information available without demanding attention. My old example of sending an invoice notice straight to the bin is not sound general advice: financial records may be needed later. Test a new filter on a small set and check where messages go.
I also moved startup notifications to Slack at the time. That suited my workflow then; it does not say what I use now or that everyone needs another app. The lasting lesson from my measurement is that less email often begins with a deliberate choice about which notices may claim your attention.