We don’t even have evidence of true mammals giving birth to live young until Late Jurassic-Early Cretaceous times.

The fact that there are surviving true mammals that lay eggs, I’m positive that most stem mammals laid eggs

Its pretty likely they did, though live birth is a possibility with how many different groups have individuals who do

Extant monotremes are known to lay eggs, and they evolved 75 million years after the last dicynodonts went extinct.