In the context of that discussion the person who posted that Paul was keeping "the law" was referring to the entire Israelite law (in this case Acts 21 was in view which included vows, sacrifices, etc.)
Adventists on the other hand generally advocate the ten commandments, but not all the law of Moses as something still to be kept. Some may also include food and hygeine laws. They do not however keep feasts, new moons, etc.
There are a minority of Adventists who do keep feasts,but they are not the norm.
When dealing with conversations that involve both MJ and SDA it is sometimes necessary to remind the SDA that they are not of the same position as the Messianics in regards to the law. So when a Messianic posts that Paul kept the law, and an SDA says "amen", they are often saying "amen" to something different than they first thought.
Adventists often think of the 10 commandments as enduring moral law, and make a vast distinction between the 10 on stone and the book of Moses.
But then when Adventists see "the law" they tend to read back in only the ten commandments. That is often not accurate.
For instance, Adventists may often quote Matthew 5 to demonstrate that the law has not passed away. But in doing so they overlook that Jesus quotes not just from the 10 commandments, but other sections of the law.
Yes, Jesus references the commandments about murder and adultery. But He also talks about oaths, eye for eye, etc.
Mat 5:18 For truly, I say to you, until heaven and earth pass away, not an iota, not a dot, will pass from the Law until all is accomplished.
Mat 5:19 Therefore whoever relaxes one of the least of these commandments and teaches others to do the same will be called least in the kingdom of heaven, but whoever does them and teaches them will be called great in the kingdom of heaven.
The words apply to the whole law, not just the ten commandments.
Murder
Mat 5:20 For I tell you, unless your righteousness exceeds that of the scribes and Pharisees, you will never enter the kingdom of heaven.
Mat 5:21 "You have heard that it was said to those of old, 'You shall not murder; and whoever murders will be liable to judgment.'
Mat 5:22 But I say to you that everyone who is angry with his brother will be liable to judgment; whoever insults his brother will be liable to the council; and whoever says, 'You fool!' will be liable to the hell of fire.
Mat 5:23 So if you are offering your gift at the altar and there remember that your brother has something against you,
Mat 5:24 leave your gift there before the altar and go. First be reconciled to your brother, and then come and offer your gift.
Mat 5:25 Come to terms quickly with your accuser while you are going with him to court, lest your accuser hand you over to the judge, and the judge to the guard, and you be put in prison.
Mat 5:26 Truly, I say to you, you will never get out until you have paid the last penny.
Jesus shows that behind the command against murder, one of the ten, is also the principle of reconciling with your brother and not hating your brother.
Adultery
Mat 5:27 "You have heard that it was said, 'You shall not commit adultery.'
Mat 5:28 But I say to you that everyone who looks at a woman with lustful intent has already committed adultery with her in his heart.
Mat 5:29 If your right eye causes you to sin, tear it out and throw it away. For it is better that you lose one of your members than that your whole body be thrown into hell.
Mat 5:30 And if your right hand causes you to sin, cut it off and throw it away. For it is better that you lose one of your members than that your whole body go into hell.
Mat 5:31 "It was also said, 'Whoever divorces his wife, let him give her a certificate of divorce.'
Mat 5:32 But I say to you that everyone who divorces his wife, except on the ground of sexual immorality, makes her commit adultery, and whoever marries a divorced woman commits adultery.
Another of the 10 is enlarged upon. Lust is adultery of the heart. Even thoughts matter.
But then Jesus turns to some issues from other commands.
Oaths
Mat 5:33 "Again you have heard that it was said to those of old, 'You shall not swear falsely, but shall perform to the Lord what you have sworn.'
Mat 5:34 But I say to you, Do not take an oath at all, either by heaven, for it is the throne of God,
Mat 5:35 or by the earth, for it is his footstool, or by Jerusalem, for it is the city of the great King.
Mat 5:36 And do not take an oath by your head, for you cannot make one hair white or black.
Mat 5:37 Let what you say be simply 'Yes' or 'No'; anything more than this comes from evil.
This command is from the OT law, but not the ten commandments.
Compare:
Num 30:2 If a man vow a vow unto the LORD, or swear an oath to bind his soul with a bond; he shall not break his word, he shall do according to all that proceedeth out of his mouth.
Lev 19:12 And ye shall not swear by my name falsely, neither shalt thou profane the name of thy God: I am the LORD.
So when Jesus speaks of the law He does not refer only to the ten.
Eye for Eye
Mat 5:38 "You have heard that it was said, 'An eye for an eye and a tooth for a tooth.'
Mat 5:39 But I say to you, Do not resist the one who is evil. But if anyone slaps you on the right cheek, turn to him the other also.
Mat 5:40 And if anyone would sue you and take your tunic, let him have your cloak as well.
Mat 5:41 And if anyone forces you to go one mile, go with him two miles.
Mat 5:42 Give to the one who begs from you, and do not refuse the one who would borrow from you.
Compare:
Exo 21:24 Eye for eye, tooth for tooth, hand for hand, foot for foot,
Exo 21:25 Burning for burning, wound for wound, stripe for stripe.
Again Jesus references parts of the law not in the ten.
Not only that, sometimes the word law means the whole first five books, and when paired with "the prophets" can indicate OT scripture. We also see this in Matt. 5:17
Mat 5:17 "Do not think that I have come to abolish the Law or the Prophets; I have not come to abolish them but to fulfill them.
Both the law and prophets are involved. Likely this is a reference to the whole of the OT scriptures.
