Ach, Markovich, 3...Bb4 in the QGD has been played (in the last decade or so) by GMs including Aronian, I. Sokolov, Onischuk, Moskalenko, Serper, Kacheishvili, P.-H. Nielsen, Nogueiras, Luther, Aleksandrov, the late Karen Asrian, Mchedlishvili, Barsov, Jonkman, Dizdar, Godena, Peralta, P. Horvath, Kovacevic, Del Rio de Angelis, Khenkin, Stripunsky, Landa, Mitkov, Teske, Lalic, Greenfeld etc. The GM list for the 4. e3 d5 Nimzo includes Kortchnoi, Ljubojevic, Rozentalis, Berg, V. Akobian, Pavlovic, Kurajica, "our own" J. Aagaard, Blatny, Nybäck, I. Sokolov, Gulko, Landa, Kogan , Gausel, Rabiega, Mitkov etc. Some of those guys played the line in question several times in that period. I would say that seems to be in line with both lines being no worse (for Black) than "+=", certainly not "±". (Your second proposed definition of "+=" sounds very much like the definition of "±".)
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