Q: Is it required that concelebrants kiss the altar before or after Mass?
A: GIRM 211 says that upon arriving at the altar, concelebrants bow to the altar and kiss it. I just today read liturgical protocols for the Diocese of Hamilton ON, where the bishop gave permission in diocesan celebrations for the priests to omit the kiss. I think it’s expected if you can do it. Although you could also argue that your concelebrants do not arrive at the altar, so they don’t kiss it.
At the end of Mass, the presider and deacon kiss the altar (GIRM 251). There’s no mention of concelebrants doing so.