I've heard that Manu laid down strict rules for the prevention of drinking. He ruled that even a seller of soma, the stuff regarded as the holiest drink by Rig-Vedic gods, does Not deserve to attend ceremonies for offering sacrifices to the gods. Is what I've heard true?
It is true that drinking liquor was heavily criticized by Manu. It was considered one of the major crimes. For example,
"Killing a priest, drinking liquor, stealing, violating the guru’s marriage-bed, and associating with those (who commit these acts) are called the major crimes." - Manu Smriti 11.55
There are other verses as well that expresses similar opinions and also prescribes punishments.
However, there is one verse, where Manu was not so strict; he did not outrightly ban it but advised not to indulge in it.
"There is nothing wrong in eating meat, nor in drinking wine, nor in sexual union, for this is how living beings engage in life, but disengagement yields great fruit." - Manu Smriti 5.56
Soma was allowed. There are many verses. I am giving two examples.
"Food of sages, milk, Soma, meat without elaborate preparation, and natural salt are, by their very nature, called 'sacrificial food'." - Manu Smriti 3.257
"A man who has stored away food sufficient to support his dependants for three years or more is entitled to drink Soma." - Manu Smriti 11.7
"If a twice-born man who possesses less resources than that drinks Soma, he will not reap its reward, even though he may never have drunk Soma before." - Manu Smriti 11.8
So you can see that drinking Soma is not punishable, but rewarding.
However, Soma sellers were looked down upon. So drinking soma is ok as part of the sacrifice, but you should not sell it as liquor.
"An arsonist or poisoner, a man who eats the food of the son of an adulterous woman, a man who sells Soma, a seafarer, a panegyrist, an oil-vendor, a false witness (these persons should be avoided)." - Manu Smriti 3.158