It's still a thing, yes. In fact, some places, like where I work, even still use some fractional T1's (which would be a T1 with a bandwidth cap on it).
In terms of data, a T1 is a [specific type of] 1.5 Mbit connection. Nothing more, nothing less, at least as it relates to modern networking. Since your question relates to "modern" networking, I should point out that if you see T1s today, you will most often see a "bundle" of T1's, which are multiple T1 lines aggregated together to increase capacity, and you get 1.5 Mbits of bandwidth for every T1 in the bundle. To your question about end users, in terms of data, you can hook your T1 up to a switch (as we do at our locations with T1's), and theoretically have as many endpoints as you want sharing the connection... but they all have to share the 1.5 Mbits of bandwidth (per T1 in the bundle).
In terms of voice, if you use a T1 (or bundle of T1's), you get the same data rate, but more importantly, the ability to digitize 24 channels of voice communications simultaneously... so a T1 for voice (which is the same technology as for data), means that you have the ability to have 24 simultaneous land-line phone calls in and/or out of the PBX it's connected to.
As to why they're still used... well, faxes are still used, and they're even older, and technically speaking, easily replaced by far superior technologies. Infrastructure has a lot of inertia, especially given the high cost of replacing it with something better. And that's to say nothing of other sources of inertia, like the fact that my bosses still actually believe that T1's are more reliable than fiber or whatever else, or prior business relationships only add to the weight behind sticking with the status quo. The fact that you can "bundle" multiple T1's together allows you to get... tolerable... data rates out of just T1's, and if you've got an ISP that is offering deep discounts on their T1 lines to squeeze some extra money out of their old infrastructure, then you can even run into situations where you can make a compelling business case for going with T1's over a newer technology.
In our specific case, we also have remote sites that are in rural areas, where the best available connections are the T1 lines that were run many years ago, so there's just no other options for a few of our sites.