Are physics conference abstracts ever screened?
Depends on the conference. Small conferences often accept most of things, at least to the poster stage.

In some fields, a conference is a place for researchers to meet, so conference papers have little significance. 

In others, conference papers are almost as important as journal papers and its really hard to get an abstract in. Check ICCV acceptance rates.. ICCV2019 accepted 25% of the papers only. 

In any case, I doubt any conference would accept an abstract without checking it first. One thing is having low acceptance threshold and another is not checking the abstracts. 

In short: it highly depends on the conference.