Well, attendance at non-Catholic (but still Christian) services – provided there was a good reason for it and the Catholic didn’t actively participate (e.g., sing, pray, play organ) – was allowed. I don’t think there was any allowance to attend non-Christian services.
As for the Passover Seder, it has been abrogated. Consider:
“It firmly believes, professes, and teaches that the matter pertaining to the law of the Old Testament, of the Mosiac law, which are divided into ceremonies, sacred rites, sacrifices, and sacraments, because they were established to signify something in the future, although they were suited to the divine worship at that time, after our Lord’s coming had been signified by them, ceased, and the sacraments of the New Testament began; and that
whoever, even after the passion, placed hope in these matters of the law and submitted himself to them as necessary for salvation, as if faith in Christ could not save without them, sinned mortally. Yet it does not deny that after the passion of Christ up to the promulgation of the Gospel they could have been observed until they were believed to be in no way necessary for salvation; but after the promulgation of the Gospel it asserts that they cannot be observed without the loss of eternal salvation” (Council of Florence,
Cantate Domino, A.D. 1441: Denz. 712).
catecheticsonline.com/SourcesofDogma8.php