Is Pat Yisrael a halacha or a chumra?

Chazal prohibited us from eating bread baked by a non-Jew (פת עכו”ם) so that we shouldn’t come too close to them and intermarry with them. There are two categories of bread in this prohibition:

1. Pat Baal Habayit (home-made bread): This refers to bread made by a non Jew for his own personal use. This bread is forbidden to eat under all circumstances, and there are no leniencies.
2. Pat Palter (Bakers bread): This refers to bread made by a non Jew for the purpose of selling to others. Included in this is all breads and baked goods that one finds in stores, and the like. This bread according to halacha is permissible to eat, at least if bread made by a Jew is not readily available (or isn’t as good), although some people are stringent and don’t eat it. We are familiar with the notion that some people are strict on only eating “Pat Yisrael”–this refers to this type of bread that although is halachicly permissible, some people are stringent not to eat it.

דעת השו”ע (סי’ קיב סעי’ ב) שפת פלטר (נחתום) מותרת במקום שאין פת ישראל מצויה מפני שהוא שעת הדחק, וכן הכריע הש”ך (ס”ק ט). ובכלל זה כל שהפת פלטר יותר יפה מפת ישראל או שהוא מין אחר (שו”ע סעי’ ה). ודעת הרמ”א שפת נחתום מותרת אפילו במקום שפת ישראל מצויה. מאידך, יש מחמירים אפילו כשאין פת ישראל מצויה, וכן הביא הכף החיים (ס”ק נו) מהאריז”ל