I have not found this to be true at all. In fact, 4.5 years ago when I started fasting, 4.5-5.5 days was pretty easy. 36 hours was nothing. Now, 4.5 day fasts are tough, and I rarely make it through one. Lately, even 36 hours fasts are tough, though I’m sure the stress of covid and all it causes can’t help.
As for lower BMR, if you lose weight, you SHOULD have a lower BMR. (You might also lose muscle mass, because you have less weight.) The issue is whether it’s lower than it could be.