Reintroduce the hidden component.

https://material-ui.com/components/hidden/

The removal of the hidden compatibility breaks backwards compatibility.
Currently the sx property makes it difficult to replicate the functionally of the hidden component.
I think it has been prematurely removed.

For example:

<Hidden smDown><Button display='flex' /><Hidden/>

becomes,

<Button sx={{ display: { xs: "none", sm: "none", md: "flex", lg: "flex", xl: "flex" }}} />

Having to list all the breakpoints in the example above isn't ideal. Hidden supported this use case well.