Uly, why don't you use "he hadn't been shaving"?

Because of FOR SEVERAL DAYS. If you don’t give a timespan, then you can say: It was obvious that he hadn’t been shaving.” But since you have несколько дней, the past perfect implies “not one time” in several days.

thank you Uly. :))))

Though it looks very difficult for me and I didn't understand it properly

Let me try to explain it better. You can look at someone and say: “You haven’t been eating” because they’re thin. Or “You haven’t been shaving” because they have stubble. Or “You haven’t been showering” because they stink. The perfect progressive already means “for several days/for some time/lately” so you don’t have to specify these. But if you DO specify for how long, you have to use the past perfect because now it means that they haven’t done something even one time (=simple tense, not progressive) during a specific timespan.

oh, I think I got it!!!
I haven't been eating/ I haven't eaten since yesterday
when I saw him last time he hadn’t been shaving/ when I saw him last time he hadn’t shaved for weeks

And these sentences are the same in some point.

Very good! That’s exactly what it means. When you “restrict” the timeframe, you use a “plain” tense to express “not once.”

They’re the same, except that with the progressive past perfect, you don’t have to specify for how long.

thanks very much

My pleasure)))