Acts 15:21 simply means there are Jews in every city who have been brought up to know the law of Moses, and some of these Jews have become Christians and have joined themselves with the Gentile believers; and the Gentile believers are urged to behave in such a way so they do not become a stumbling block to any of these Jews while in their company in the ...
The word translated "for" in this verse means giving a reason, a therefore, a because of.
γάρ gár, gar; a primary particle; properly, assigning a reason (used in argument, explanation or intensification; often with other particles):—and, as, because (that), but, even, for, indeed, no doubt, seeing, then, therefore, verily, what, why, yet.
Churches existed apart from synagogues. Recognizing the distinction helps clarify the meaning of this verse.
It is not expecting Gentiles to adopt Jewish laws over time. The first requirement for someone to follow Old Covenant laws was circumcision.
This was never a requirement for Gentile Christians and was vehemently opposed by the Apostle Paul.