It’s true that the doctrines of salvation and the eternal afterlife (both for the saved and the damned) developed quite a bit in the inter-testamental period, and then all the more so in the New Testament. But there were also many clear signs or precursors of it (some remarkably explicit) all along. The book of Job seems to allude to an eternal consciousness in a resurrected body in some sort of paradisal state with God:

Job 19:25-27 (RSV) For I know that my Redeemer lives, and at last he will stand upon the earth; [26] and after my skin has been thus destroyed, then from my flesh I shall see God, [27] whom I shall see on my side, and my eyes shall behold, and not another. (cf. 14:12-15)

The prophet Isaiah (8th c. BC) taught similarly:

Isaiah 26:19a Thy dead shall live, their bodies shall rise. O dwellers in the dust, awake and sing for joy! . . . 

This is hardly a shadowy temporary existence in Sheol and then annihilation. But there is much more, too:

2 Kings 2:11 And as they still went on and talked, behold, a chariot of fire and horses of fire separated the two of them. And Eli’jah went up by a whirlwind into heaven. (cf. Gen 5:24: Enoch)

Psalms 16:10-11 For thou dost not give me up to Sheol, or let thy godly one see the Pit. [11] . . . in thy right hand are pleasures for evermore.

Psalms 21:4  He asked life of thee; thou gavest it to him, length of days for ever and ever. . . . (cf. 23:6)

Psalms 49:7-9, 15 Truly no man can ransom himself, or give to God the price of his life, [8] for the ransom of his life is costly, and can never suffice, [9] that he should continue to live on for ever, and never see the Pit. . . . [15] But God will ransom my soul from the power of Sheol, for he will receive me. . . . 

Psalms 73:24, 26 . . . afterward thou wilt receive me to glory. . . . [26] . . . God is the strength of my heart and my portion for ever.

Daniel 7:18 But the saints of the Most High shall receive the kingdom, and possess the kingdom for ever, for ever and ever.

Daniel 12:2 And many of those who sleep in the dust of the earth shall awake, some to everlasting life, and some to shame and everlasting contempt. 

The notion of spiritual salvation is present in the Old Testament as well:

Psalms 34:6-7 This poor man cried, and the LORD heard him, and saved him out of all his troubles. [7] The angel of the LORD encamps around those who fear him, and delivers them. 

Psalms 51:1-3, 5, 7, 9-14 Have mercy on me, O God, according to thy steadfast love; according to thy abundant mercy blot out my transgressions. [2] Wash me thoroughly from my iniquity, and cleanse me from my sin! [3] For I know my transgressions, and my sin is ever before me. . . . [5] Behold, I was brought forth in iniquity, and in sin did my mother conceive me. . . . [7] Purge me with hyssop, and I shall be clean; wash me, and I shall be whiter than snow. . . . [9] Hide thy face from my sins, and blot out all my iniquities. [10] Create in me a clean heart, O God, and put a new and right spirit within me. [11] Cast me not away from thy presence, and take not thy Holy Spirit from me. [12] Restore to me the joy of thy salvation, and uphold me with a willing spirit. [13] Then I will teach transgressors thy ways, and sinners will return to thee. [14] Deliver me from bloodguiltiness, O God, thou God of my salvation, and my tongue will sing aloud of thy deliverance. . . .

Proverbs 15:24 The wise man’s path leads upward to life, that he may avoid Sheol beneath.

Isaiah 45:17, 21-22 But Israel is saved by the LORD with everlasting salvation; you shall not be put to shame or confounded to all eternity. . . . [21] . . .  a righteous God and a Savior . . . [22] “Turn to me and be saved, all the ends of the earth! For I am God, and there is no other.

Jeremiah 17:14 Heal me, O LORD, and I shall be healed; save me, and I shall be saved; . . .

Jeremiah 31:33-34 But this is the covenant which I will make with the house of Israel after those days, says the LORD: I will put my law within them, and I will write it upon their hearts; and I will be their God, and they shall be my people. [34] . . . for I will forgive their iniquity, and I will remember their sin no more. 

Most of these passages likely refer to “physical salvation” from enemies and death, but some may have a double application, and some may refer to spiritual salvation only.

The idea of being abandoned to Sheol is similar to eternal hellfire (a developmental precursor of it). Thus, eternal life or eschatological salvation is described as “thou dost not give me up to Sheol” (Ps 16:10). God delivers or rescues the righteous from Sheol (“he brings down to Sheol and raises up”: 1 Sam 2:6; cf. Ps 30:3; 49:15; 86:13; 89:48). Sheol is the hopeless final state of the wicked (Ps 6:5; 9:17; 31:17; Is 14:11, 15 cf. Mk 9:48; 38:18; 66:24)

Jeremiah 15:14 . . . in my anger a fire is kindled which shall burn for ever. (cf. 17:4; Dt 32:22)

Daniel 12:2 And many of those who sleep in the dust of the earth shall awake, some to everlasting life, and some to shame and everlasting contempt.

Many references to “the pit” also refer to Sheol.