‘And as they went on their way through the cities, they delivered to them the decrees to keep which had been ordained of the apostles and elders who were at Jerusalem.'
And as they went through the cities they passed on the details of the decisions made in Jerusalem, with Silas there to confirm them. This is in fact the only time that these are referred to as such. The issue of food offered to idols, which would always be offensive to Jews under any circumstances, Paul deals with elsewhere in more detail (e.g. 1 Corinthians 8) when he softens its application, and clearly idolatry and fornication were always forbidden. The question of strangulation of meat was a fellowship matter and simply never seems to have arisen as a problem. This would suggest that it was observed where it was felt necessary out of Christian consideration, as was right.
It should be noted that this was the advice of the enquiry sent to churches in areas very much involved with Jewish connections. It is actually nowhere said to be binding on all Christians. Thus among these churches its requirements were very necessary. It would be different among fellowships where Jewish Christians were rare, although we would always expect Christians to take into account people's idiosyncrasies. It may well be because Paul did not want the advice to become a ‘decree' that he never mentions it in his letters, even when the issues arise.