How do you handle milestone sign-offs on client projects?

I'm a web developer, designer who mostly does client work. When I deliver a milestone, let’s say, the design phase, or a staging build, how do I formally get the client to sign off so that I have something concrete to point back to if they change their mind later?

Right now I send the staging URL on slack or WhatsApp and wait for a response. Sometimes they say "looks great." Sometimes they go quiet for a week and then say "this isn't quite right" after I've already moved on.

I've tried email trails. I've tried shared Notion docs. Neither of them create a genuine "this version is now closed" moment.

Curious what other devs do.