In my younger days the priest would remind us that missing Mass on Sundays or Holy Days was a mortal sin. I don’t hear this anymore. Is it still a teaching of the Church?

Yes, the Catechism of the Catholic Church says, The faithful are obliged to participate in the Eucharist on days of obligation, unless excused for a serious reason (for example, illness, the care of infants) or dispensed by their own pastor. Those who deliberately fail in this obligation commit a grave sin (# 2181)
 
The Lord Jesus also warns, If you do not eat the flesh of the Son of Man and drink his Blood, you have no life in you (Jn 6:53) Thus, to miss Mass and stay away from Holy Communion is a form of spiritual starvation. Further, we fail to give God the praise, worship and thanksgiving He is due.
 
It is a sad fact that this precept and moral teaching is underemphasized today. Priests, catechists and parents must be clearer in teaching and witnessing to this requirement rooted in the Third Commandment. They must also teach why.
 
There is a modern tendency, not wrong itself, to emphasize “positive” reasons to do things rather than simply quoting laws. But the gravity of the offense against God’s Law should not go unstated. Further, obedience to God’s law is of itself good, and brings with it many benefits and blessings such as the instruction in God’s Word at Mass and the astonishing blessing of being fed on the Lord’s Body and Blood.