Right now, selectors after :host are not properly shimed (see #5390). Users already use this feature for theming purposes.

If we supported the /deep/ and >>> selectors, users could just replace :host ... with :host >>> ....

Semantics:

a >>> b

Will be converted into

a[ngcontent_xzy] b

I.e. the selector after >>> and /deep/ will not be shimmed.