It’s the symbol to the left of the URL > Show Certificate. They even make it available on iOS Safari (Page Info > Connection Security Details), but if it’s expired, you’ll know by the big red warning page.
I don't have that :-) — what I see when I click the thingamajig on the left side of the URL is a menu with "Hide Distracting Items", "Zoom", "Find" and "Website settings".
For me it’s hidden behind a kebab menu in that thingamajig you described.
Well, either it isn't there in my Safari (macOS 15.7.2) or I can't find it.
I have found "Connection Security Details…" in the "Safari" menu, though. But my point still stands: average users won't see any certificate information without serious effort.
Ah no, i am on iOS 26.
In Safari, I don't even know how to find that information anymore. When I want to check expiration dates for my own sites, I start Firefox.