Google Books statistics indicate that the use of the expression "if you should" and especially of the expression "if you shall" per unit of text length dramatically and steadily declined since the 1700s, as you can see in this graph. As compared to the 1700s, the former expression is nowadays used about 7 times less frequently, and the latter expression - about 50 times less frequently.
Why did this happen?
My question is not meant to be broad or a request for subjective opinions. Instead, I mean to ask whether there are any obvious objective factors that explain or could explain such a decline, such as a documented change in the meaning or connotation. I am not a native speaker and do see any possible factors at all, so I hope that native speakers can shed some light.
My understanding is that the expression "if you shall/should" roughly means "if you have/had a duty or obligation to," and I am puzzled as to why the expression became so much less used to express the idea. I see nothing wrong or weird in saying, "I will help you find another apartment and relocate to it if you shall vacate your current landlord's property by the end of this month."
In an attempt to find the answer, I looked at statistics for the expressions "you should" and "you shall" (i.e., without "if"). I found that although the use of the latter expression considerably declined since the 1700s (by a factor of 10), the use of the former one did not decline at all: Graph. Thus, the statistics of "you shall/should" cannot explain the magnitude of the decline of the use of "if you shall/should." And I even do not see why the expression "you shall" became less frequently used than before.
I am puzzled and humbly hope to read enlightening responses by native speakers.
Update: As pointed out by JK2 in an answer below, the statistics are different if we replace "if" by "If" (i.e., if we make the first letter capital):
The use of "If you should" (with the capital letter I in "If") is nowadays about 4 times less frequent per unit of text length as compared to the middle of the 1800s.
The use of "If you shall" (with the capital letter I in "If") is nowadays about 20 times less frequent per unit of text length than in the 1700s and 1800s.
Thus, the decline for these phrases with the capital letter I is less dramatic than for the same phrases with the small letter i, but is still very considerable.