(13) If, then, you obey the commandments that I enjoin upon you this day, loving Adonai your God and serving Adonai with all your heart and soul, (14) I will grant the rain for your land in season, the early rain and the late. You shall gather in your new grain and wine and oil— (15) I will also provide grass in the fields for your cattle—and thus you shall eat your fill. (16) Take care not to be lured away to serve other gods and bow to them. (17) For Adonai’s anger will flare up against you, and Adonai will shut up the skies so that there will be no rain and the ground will not yield its produce; and you will soon perish from the good land that Adonai is assigning to you. (18) Therefore impress these My words upon your very heart: bind them as a sign on your hand and let them serve as a symbol on your forehead, (19) and teach them to your children—reciting them when you stay at home and when you are away, when you lie down and when you get up; (20) and inscribe them on the doorposts of your house and on your gates— (21) to the end that you and your children may endure, in the land that Adonai swore to your fathers to assign to them, as long as there is a heaven over the earth.
The commentary in the present-day Reconstructionist Kol Han'shamah aptly describes the problem in this paragraph: Its detailed description of the "bountiful or devastating consequences of Israel's collective relationship to the mitzvot...offers a supernatural theology that many contemporary Jews find difficult." Simply put, it presents a doctrine of reward and punishment that most liberal Jews have found problematic, if not offensive. It has therefore been removed from most liberal prayer books in the modern era.
David Ellenson, My People's Prayer Book, Vol. 1
My own approach combines two doctrines of the Rabbis. I too admit that in the end, we cannot fathom God's justice: whether we are talking about individuals or communities, it is simply not true that the righteous always prosper and the wicked suffer and I do not know why that is. I also believe that "the reward of performing a commandment is [the propensity and opportunity to perform another] commandment, and the result of doing a wicked thing is [the propensity and opportunity to do another] wicked thing" (M. Avot 4:2). That is, we should do the right thing because it is the right thing and not out of hope for reward, and we should avoid evil acts because they are evil and not out of fear of punishment.
This approach is a far cry from the direct, reward-and-punishment thinking of the second paragraph of the Sh'ma, and yet I recite the Sh'ma each day because it proclaims God's justice, and justice must be a critical element in the God I affirm. The calculus of reward and punishment articulated in this paragraph may be too simple and ultimately inaccurate, and, for that matter, it may be immoral in the first place to do the right thing and avoid the wrong out of concern for consequences...The Rabbis too had problems with the doctrine of justice announced in this paragraph, but they included it anyway because they too had a deep faith in the ultimate justice of God as the metaphysical backdrop and support for human acts of justice.
Elliot N. Dorff, My People's Prayer Book, Vol. 1
"...In the first trial-draft of Mishkan T'filah (2002), which included the second paragraph as an option, noting that 'traditional Reform thinking challenges Deuteronomic theology, that bad events which occur are a result of communal sinful behavior. We do accept responsibility for social and natural ecology: how we treat one another and our environment has a powerful, direct impact on society and the planet.'
Nonetheless, the verses describing an angry God 'sealing up the heavens' were printed in smaller type, acknowledging the theological difficulties with this image. The paragraph ultimately remained sufficiently problematic to require both a recommendation from the Siddur Editorial Committee and a vote by the CCAR Executive Committee. Both decided to uphold the earlier Reform deletion of the paragraph in its entirety: even though subject to mitigating non-literalist interpretations, the text itself remains difficult for a modern Jew to recite in the liturgy --although the second reading on page 235 (117 Shabbat edition), 'If we can hear the words from Sinai,' by Rabbi Richard Levy, is in fact a contemporary interpretation of the second paragraph of the Sh'ma.
The third paragraph of the Sh'ma is variously referred to by the Rabbis as parashat tzitzit, since it deals with the obligation to wear fringes on the four corners of one's garment that are to serve as reminders of God's commandments, and as Y'tziat Mitzrayim, 'the Exodus from Egypt,' since at the end it refers to God as the One who redeemed the Israelites from Egypt.... When the custom of wearing a tallit during morning prayers became less frequent among Reform Jews, this paragraph came to be dropped from the liturgy (...the Berlin Reformers, Einhorn, and the Union Prayer Book were typical of this trend; the 1918 revision of the UPB reinstated a portion of the final part of the paragraph, beginning L'maan tizk'ru the full conclusion was reinstated in Gates of Prayer, 1975). Now that many Reform Jews wear tallitot at morning prayers, Mishkan T'filah has restored the entire third paragraph as an option for those who wish to recite it--but only in the morning, since the tallit is not worn at night." -- Sarason, Rabbi Richard S. Divrei Mishkan T'filah: Delving into the Siddur. CCAR Press, 2017. p 42-43.