What are the strange metals, that is briefly described in the link given in your question and the link given by @Custer. Besides this, some points to address your query are as follows:
Strange metals are a class of materials (in their metallic phase) that exhibit unusual properties at high temperatures and/or under high pressure. These materials deviate from the predictions of traditional models of condensed matter physics (like the Landau's Fermi liquid theory, where the solid is described as a sea of Fermi quasi-particles, which are excitations of the original fermions that behave as if they are non-interacting) and these strange metals are generally characterized by their high electrical and thermal conductivity, as well as some other anomalous properties.
Some examples of strange metals include high-temperature superconductors (Landau's Fermi liquid theory describes the behavior of a system of interacting fermions at low temperatures, so it's breakdown should be somewhat intuitive, atleast at the basic level), heavy fermion compounds, and certain types of quantum critical systems.
Are they something that we have really made, or just a theoretical idea?
Experimentalists have been able to produce these materials in the lab. Very few them includes:
Electron doped Cuprates [or high-temperature copper oxide (cuprate) superconductors]: The Strange Metal State of the Electron-Doped Cuprates
Richard L. Greene, Pampa R. Mandal, Nicholas R. Poniatowski, and Tarapada Sarkar
Annual Review of Condensed Matter Physics 2020 11:1, 213-229 .
Strange metal and Topological Liquid near Quantum Critical Point: Oh, E., Yuk, T., & Sin, S. J. (2021). The emergence of strange metal and topological liquid near quantum critical point in a solvable model. Journal of High Energy Physics, 2021(11), 1-23 .
Edwin W. Huang, Ryan Sheppard, Brian Moritz, Thomas P. Devereaux, Strange metallicity in the doped Hubbard model, Science 22 Nov 2019: Vol. 366, Issue 6468, pp. 987-990 .
Rodriguez, J. P., & Lederer, P. (1996). Confinement of spin and charge in high-temperature superconductors. Physical Review B, 53(18), R11980 .
Tikhonov, K. S., & Feigel’man, M. V. (2020). Strange metal state near quantum superconductor-metal transition in thin films. Annals of Physics, 417, 168138 .
If they have been prepared in laboratory, then what are their properties (or if they are still a theoretical proposition only, then what are their predicted properties)? And what could they be used for?
I've already said about the major properties of strange metals earlier.
Now the prospective uses: These substances are of great interest to scientists and researchers because of their unusual properties and the potential insights they may provide into the fundamental nature of matter and the laws of physics. However, their current applications are mostly limited to scientific research and theoretical studies.
Some potential future applications that have been proposed for strange metals may include:
- High-temperature superconductivity (some references on this have been cited in the previous answer; mainly their high electrical conductivity is useful here)
- Quantum computing (high electrical and thermal conductivity of strange metals may be useful in the development of quantum computers)
- Other prospective applications in various fields of Science & Technologies (such as in aerospace technology, energy storage, quantum sensors...) may be seen in future.
But note that these applications are not so successful yet, may be we see them in our long run. They are still just a matter of research and a spectacle for the material scientists to peep into the details of the fundamental nature of matter more vividly, as I mentioned earlier.