Has the church come out with an official pronouncement prohibiting infant baptisms during Lent?
No. However, the Church has tried to place a greater emphasis on the penitential nature of the season of Lent (which has fallen into considerable neglect in recent years). Individual Bishops or pastors may choose to refrain from the routine practice of “celebratory” Sacraments during this season (except, of course, in matters of urgency).
At many Parishes, Lent has become “business as usual, except wearing purple.” It is entirely within the bounds of pastoral discretion to abstain from celebrations which necessity does not dictate.
However, one could argue that the possibility exists that an infant born during Lent could unexpectedly die before Easter, without recourse to Baptism, and this risk (however small) creates necessity (due to the uncertain fate of unbaptized infants). This is a reasonable conversation to have with the bishop or priest who imposes this restriction.
You could ask, “what happens if my unbaptized baby dies before Easter?” (Correct Answer: we don’t really know). Second question: What happens if my baby is baptized today (even though it’s Lent) and dies tomorrow? (Correct Answer: s/he goes straight to heaven). Final question: Can you absolutely guarantee that my baby will not die before Easter, without recourse to Baptism? (Correct Answer: No, duh). That should present a fairly convincing case.
It’s good to re-emphasize the penitential nature of Lent. But, IMHO, it’s misguided to do so at the risk of human souls (however small that risk may be).