T1 is still used a lot internally for connecting to existing phone PBX's that might not have an Ethernet connection at all.

We still sell and use them (small B2B focused clec/isp in Oregon). We use them fairly commonly as dedicated voip links, or for PRI phone systems where the customer insists on avoiding voip. Also in some places they are about the best connectivity you can get when the only alternatives are way oversold dsl or trying to aim a wisp antenna though some trees to catch a signal bounced off a mountainside. And, if you have the means, (and ILEC cabling facilities), the circuits can be bonded together to arbitrary numbers. So, yeah, it's slow. But it's better than nothing. That said I fully expect Starlink to take that ultra-rural business in the coming year or two.

I was still using one until recently. We do inbound phone support and had 24 lines on a T1, because I wanted to make sure I had SLA'ed last-mile connectivity. Previous iteration of this was a 24 line PRI directly into a Sangoma T1/SIP Gateway. After a move + installation, I had an Adtran device that was IP based. It is my understanding that that it was more or less VOIP over a data T1, and the on-premise Adtran device turned it back into a voice T1 for the benefit of our T1 Gateway.

Recently I've switched it to a metro fiber ring + SIP trunking.

It's surprisingly difficult to get SLA'ed SIP trunking in the SMB market.

A few years ago we had one put in for a connection to VisaNET. ISO8353 messages are really small so 1.54Mbps was plenty fast for the number of transactions we were sending.

I'm trying to remember the exact number, but IIRC it was good for a couple of thousand transactions a second.

still had one as of a year and a half ago, for bringing in phone lines to a business. they'd converted it a year previous to be SIP over an IP link, though. meh. at that point it was only a tiny smidge better than twilio over business comcast, and was waaaay more of a hassle.