How beneficial is breastfeeding past 1 year in terms of antibodies for baby?

Depends. The actual benefit from antibodies begins to be diluted when solids are introduced. By 8-9 months, with most common feeding patterns, the antibody benefit is minimal. If the kid is "nurture needy" and lingers at feeding, the benefit from that point remains.If the kid is in a hurry to finish and gets off quick, it is a good time to wean.
Benefecial enough. Difficult to say how much benefecial, but there are immune benefits, dental and possible IQ benefits continued nursing. It also likely helps prevent ovarian and breastcancer in the mother, but the research results are mixed. It would also save $$ due to less formula for the infant. Despite advertising commercial infant formula will always be second rate to mother's milk.