It's actually a really cool concept honestly. I do this on a very tiny scale at home. I run a small server cluster largely as a playground but also for things like Plex and Vaultwarden. The waste heat from that is pushed out the back of the rack and a heat pump water heater a few feet away uses that to help heat water.
As you said, seems like there are many opportunities to do this on a much lager scale.
It is being used in some places actually:
https://reasonstobecheerful.world/data-center-heat-green-energy/
https://www.greenbiz.com/article/data-center-warms-homes-waste-heat
https://www.weforum.org/agenda/2022/08/sustainable-data-centre-heating/