How to change styles of child mat angular component?

As angular official documentation says, ::ng-deep , >>>, /deep/ is deprecated and will be removed soon:

https://angular.io/guide/component-styles#deprecated-deep--and-ng-deep

If i am using mat components like <mat-checkbox> or a more comprehensive one like <mat-table> how could I have some changes to that component from parent?

1. Should I cancel view encapsulation for that component and write styles in .SCSS files?
2. How do I edit styles of inner material angular component if deep selector is going to be removed?
3. What is the proper way to do that?

As the mention document says you can use the combination of ::ng-deep with :host and it will be OK in this way.

In order to scope the specified style to the current component and all its descendants, be sure to include the :host selector before ::ng-deep. If the ::ng-deep combinator is used without the :host pseudo-class selector, the style can bleed into other components

:host /deep/ h3 {
  font-style: italic;
}

But, you also can use the custom CSS class & id to apply your custom css on .CSS or .SCSS files on the Angular Material Components. using .class & #id in combination with mat default classes works. In addition you can use custom Angular material classes in your componnent style files ( .CSS or .SCSS) to override the like this:

.app-component-style {
/* All the CSS here */
.mat-tab-group .mat-tab-label {color: green;}
}

So, keep using it as Dudewad mention here too: https://stackoverflow.com/a/49308475/4185370