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Some cities do use big heat pumps in district heating systems, so probably it could be used for steam too.

I think steam is somewhat antiquated, it has needlessly large temperature differential to the environment which leads to excess transfer losses compared to normal district heating using hot water.

Yes. There are heat pumps capable of producing steam up to 200C. They are meant for industrial applications, and I think they're a relatively new development so not much used yet.

The efficiency goes down significantly with those temperatures, given that most sources of waste heat are low-temperature (such as datacenter heating water, waste water, etc - places where it makes economically and ecologically sane to extract heat).

That's the problem even in district heating where temps are ~115C (like in Helsinki). Sadly, reducing the temperature is not an easy task, since many buildings have heating systems designed for higher temps.

Even lowering those temps to 80C would improve efficiency of the heat pumps a lot. But it's a big project to get it done and will most likely take a decade or more.

Some cities do use big heat pumps in district heating systems, so probably it could be used for steam too.