In short, the Covenants are for Israel (Ex 19, Jer 31, Rom 9, the New Covenant yet still future and the Old still passing away in that age, Heb 8:13) and the plan for the kingdom and the land were what the Apostles were anticipating (and promising) if Israel repented (Acts 1 , Acts 3). Try preaching that today.
Gentile believers in the Acts Age (after being grafted into the blessings of Israel to make unbelieving Israel jealous, Rom 10, Rom 11) were put under the Levitical law for Gentiles living among Israel (Lev 17, Acts 15, Acts 21). Under the Law, a Gentile could live peaceably among Israel, but could not participate in the Feasts unless a proselyte (circumcised). So, the Law has instructions for Israel and for Gentiles living among Israel. It is no guide for today in that way.
In the Acts Age, believing Jews were still circumcising their boys and Paul was obeying the Law (Acts 21). So, the quick death of Ananias and Sapphira (Acts 5) is in keeping with that economy and not our current economy. We do not see people dropping dead from lying to the Holy Spirit or holding back pledged money to a "church."
In that Age, Paul was still keeping part of the non-sacrificial Law and he was accused of not following the Law.
“You see, brother, how many myriads of Jews there are who have believed, and they are all zealous for the law; but they have been informed about you that you teach all the Jews who are among the Gentiles to forsake Moses, saying that they ought not to circumcise their children nor to walk according to the customs.
-Acts 21:20-21
But Paul certainly was keeping the Law. He was upset that anyone thought differently. The accusations were "nothing" (false), he pleads. And he was clear to tell Gentiles that they did not have to keep the Law, just the "necessary things" gleaned from Leviticus as noted.
Take them and be purified with them, and pay their expenses so that they may shave their heads, and that all may know that those things of which they were informed concerning you are nothing, but that you yourself also walk orderly and keep the law. But concerning the Gentiles who believe, we have written and decided that they should observe no such thing, except that they should keep themselves from things offered to idols, from blood, from things strangled, and from sexual immorality.”
-Acts 21:24-25
One law shall be for the native-born and for the stranger who dwells among you.
-Ex 12:49
When they asked him to stay a longer time with them, he did not consent, but took leave of them, saying, “I must by all means keep this coming feast in Jerusalem; but I will return again to you, God willing.” And he sailed from Ephesus.
-Acts 18
But neither the twelve nor Paul required Gentile believers to keep any feasts. just the "necessary things" in Acts 15 and 21 gleaned from the Law for Gentiles. The Lord's Supper was the Passover (the Lord said so) and those who took it unworthily got sick or died. Is that happening today? I don't think so. And in the Law, Gentiles could not partake unless circumcised (Ex 12:48).
As we see in the Book of Hebrews, the Law (apart from the sacrificial Law) and the Old Covenant were still recognized by the churches of the Acts age (Heb 8, Heb 10). When Paul is given his revelation (unknown to the sons of men before that, Eph 3), the middle wall between Jew and Gentile came down and the law seen in the ordinances which separated Jew from Gentile was done away (Eph 2).
When we understand these conditions, the swift punishment of Acts 5 (even before the grafting in of Gentiles into Israel's blessings), with the hope of the restored kingdom in view (Acts 1), and the future rulers ready to take their places on the twelve thrones (Matt 19), while facing a cleansing and winnowing coming for Israel, and the reward of the New Jerusalem and the implementation of the New Covenant (prophets and Rev)... we can see Ananias and Sapphira suffering under those conditions (conditions we do not see today).
Looking at Acts 5:3, the Geneva Bible makes a rather stunning argument:
For when they had appointed that farm or possession for the Church, they were foolish to keep away a part of the price, as though they were dealing with men, and not with God, and therefore he says afterwards that they tempted God.
I don't see a lot of people dropping dead these days for not honoring promises to the "Church." When we understand that not everything called a "church" is the same gathering, we can start to see God's different dealings with different called-out groups for different purposes.
Pentecost was anticipating the Millennium and the conditions of that age to come, but it was not the start of that age. And the plan in sight was the earthly plan for the nation of Israel in the land of promise under the restored Kingdom (Acts 1, Rom 9, etc.).
`He that worketh deceit shall not dwell within My house: he that telleth lies shall not tarry in My sight. I will early (morning by morning or every day) destroy all the wicked of the land; that I may cut off all wicked doers from the city of the LORD'.
-Psalm 101:7-8
The Lord warned at His return in Luke 21 that after Israel's time of testing and cleansing, the wicked will we dealt with swiftly. But Israel mist go through that winnowing period before a Virgin Israel (New Covenant, Jeremiah 31) can realize the fullness of God's plan.
“And there will be signs in the sun, in the moon, and in the stars; and on the earth distress of nations, with perplexity, the sea and the waves roaring; men’s hearts failing them from fear and the expectation of those things which are coming on the earth, for the powers of the heavens will be shaken. Then they will see the Son of Man coming in a cloud with power and great glory.
-Luke 21:26-27
There will be direct judgment for sin each day in the Millennium so that God's kingdom standards can be maintained. In this age of grace such judgment is suspended. If this were not so, the undertakers would have a real problem. The death judgment of Ananias and Sapphira is not the only judgment in the Acts. Later on is recorded the judgment of blindness that fell upon Elymas [Acts 13 by Paul].
-Stuart Allen on the Acts Age and Afterwards (excerpt)
For he who eats and drinks unworthily, eats and drinks damnation to himself, not discerning the Lord’s body. For this reason many are weak and unhealthy among you, and many die.
-1 Corinthians 11:29-30
So, some in the Acts age dropped dead for taking the Lord's supper "unworthily" and some dropped dead for lying to the Holy Spirit and holding back. All believers in that age either obeyed the non-sacrificial Law (Jews) or the Law for Gentiles living among Israel (the "necessary things"). None of those conditions exist in this current present age. No one is dropping dead (or getting sick) for taking the Lord's Supper unworthily, and no one is dropping dead for not keeping a pledge to the church.