And some companies continue to do so! It's not a lost technology. But the way it typically works is that big contracts come with a ops engineer or two to live with the customer and help them set it up.
At some companies, this is clearly demarcated from the SWE role - at others, stereotypically the "IBM mainframes, SAP and Oracle DBs" category the OP mentions, they kinda shade into each other. Which is, of course, why those technologies have such negative reputations. (There's also the Palantir route, where you have a clear demarcation but lie about it to new grads who don't fully understand the conflict between "forward deployed" and "software engineer".)
At some companies, this is clearly demarcated from the SWE role - at others, stereotypically the "IBM mainframes, SAP and Oracle DBs" category the OP mentions, they kinda shade into each other. Which is, of course, why those technologies have such negative reputations. (There's also the Palantir route, where you have a clear demarcation but lie about it to new grads who don't fully understand the conflict between "forward deployed" and "software engineer".)