Since ‘no’ one will be justified by the ‘works’ of the Law, so this must the part which “died” or “ceased” from ‘practice’ as even Acts 15’s conclusion reveals —> simply because by doing the “works of the Law”, ‘no’ one will be justified (so, “no” point in doing it further —> thus it makes sense to ‘cease/die to it’, as Romans 3:20 quoted below states).

Note that the ‘knowledge of sin’ remains with the Law; hence, we don’t sin against the non-ritualistic part —> Chiefly, the Ten Commandments:

“because by the works of the Law no flesh will be justified in His sight; for through the Law comes the knowledge of sin.” (Romans 3:20)

Note that the verse above points out two ‘different’ things about ‘The Law’:
—> (i) ‘works of the Law‘ (“no” justification)
—> (ii) ‘knowledge of sin‘ (remains till one dies, as the verse below explains):

“What then shall we say? Is the Law sin? By no means! For I wouldn’t even have been aware of sin, if not for the Law. For example, I would not have known what coveting was if the Law had not said, “Do not covet.” (Romans 7:7)

“But now we have been released from the Law, having died to that which we were bound, in order for us to serve in newness of the Spirit, and not in oldness of the letter.” (Romans 7:6)

Note that the phrase ‘released from the Law’ —> must mean ‘freed from its ritualistic observances’, —> since ‘having died to that (rituals – letter- of the -Law) which we were bound’, right?

As for the Sabbath, it’s kept in its “first form” (without its ritualistic-rules & even mandatory sacred assemblies) which is —> ‘keep it holy & don’t work (rest)’, see Exodus 20 for this ‘first definition’.

That is to say that the Law helps us identify “what is sin” or ‘not’ —> ‘knowledge of sin’ —> but its “works” (ritualistic) are ‘nullified’ (offer “no” justification).

Another key point is to observe the last few words of this verse, namely, “if righteousness comes through the law, then Christ died in vain.” (Galatians 2:21) —> the part of the law which ‘added righteousness’ was ‘obeying its ritualistic’ parts —> So, again this is what that does “not” add any righteousness when we have come to Christ.

Note that obeying any part of the Ten Commandments, say for example, ‘do not murder’ —> does “not” add any righteousness to you —> but rather ‘keeps you away from sin’ for you ‘recognize it as a sin’ —> “knowledge of sin” —> Hence, it does ‘not’ nullify Christ’s work in any way.

That’s why I’ve maintained (by my understanding) that the “ritualistic” (‘works’) of the Law is nullified (for it produces ‘no’ righteousness) but the ‘knowledge of sin’ (from the Ten Commandments/concepts/principles of the Law) helps us ‘identify’ what could be a ‘sinful act’ and help us thus to ‘avoid it’.