Being able to set location seems to have disappeared from the mobile web interface. I'm sure it used to be there as a field on the "Write" dialog providing your browser had the right user agent set.
I really don't understand why Google makes it so hard for non-Android or IOS devices (like laptops) to set locations, view nearby posts, add location to posts and so on. It's quite a long time now since Latitude was closed down and I really thought these capabilities would have been added by now. For this post in the desktop interface, I can attach a photo, link, video, event. Why can't I attach a location?
As far as I can tell that leaves me with no method of setting current location on my profile.
[edited to add] This seems like it might be a Chrome thing. Using Firefox and setting user agent to Chrome on iOS, the Check in and Location options are visible again.
I think all that happened and what prompted the original post is that Google's list of acceptable user agent strings had changed slightly.