I can't for the life of me figure out why people in America (in E"Y I have seen people being makpid) who are totally not makpid on this. Why!? It's an explicit halacha in Shulchan Aruch!
Rav Yitzchok Berkowitz is quoted as saying that if there is a Mechitza or any type of bench or chair which is at least 10 tfachim high (and with no spaces within it wider than three tfachim) than it is 100% mutar to take steps backwards, because its considered as if you are in a separate area than the mispallel.
Furthermore, I saw in a sefer from a talmid of Rav Dovid Feinstien (Vedibarta Bam) a limmud zchus that says even if it's not halachically considered a separation, ie. not 10 tfachim high, its still mutar, because you don't need a separate reshus to take steps back, instead its enough a minor separation between each other, thus even a shtender or table is enough of a separation. Although Lmaaseh R Dovid feels that one needs a full 10 tfachim
The halacha is also that if one needs to go to the bathroom it is mutar to step back.
Parentheically, the Aruch Hashulchan says that this halacha is not dependent on whether it is distracting to the mispaellel, rather it is assur because you are standing in "his presence of G-d" ie. his 4 amos. Thus, even if the persons face is covered with a tallis and can't see you step back is still assur.