One can still find the id, even when a person has chosen a username that masks it. Right click the block option of a user you are not friends with, copy the link and voila, you've got the user id now.

I mostly use that to see public but normally inaccessible pictures of people I am not friends with (i.e. pictures they've been tagged in).

Try it yourself (for some reason this only works on desktop mode): https://www.facebook.com/search/{$PUT_THAT_ID_HERE}/photos-o...