Depends on what you mean by "stop using". We never even had Internet at the house I grew up in, but for at least one job around 2000, we had dial-up on standby in case the ISDN went down, and occasionally used it for side projects even when the ISDN was working. (In fact I'm not sure we ever needed to fail over in the time I was there.). One of those side projects was mine, which means that ~2000 was the first and last time I was a dial-up user.

But then there's provisioning dial-up, which is kind of using it from the other end ...iiif you squint a bit. In that case people were still occasionally signing up with another company I worked for circa 2014. I could probably have found the usage stats back then, but was never curious enough to check and never had the need to, and I've since moved on.

Best as I can tell, that company no longer offers sign-ups to old-school dial-up service. Can't say I'm surprised. I do wonder if they've any old accounts grandfathered in though. I don't remember the dial-up number to check if there's something modem-y on the other end.