It seems his wife Winnie was openly advocating for it while Mandela was in prison but I can’t find him associating with it.

Necklacing was used by the black community to punish its members who were perceived as collaborators with the apartheid government.[2] Necklacing was primarily used on black police informants; the practice was often carried out in the name of the struggle, although the executive body of the African National Congress (ANC), the most broadly supported South African opposition movement, condemned it.[3][4] In 1986, Winnie Mandela, then-wife of the imprisoned Nelson Mandela, and who herself had endured torture and four imprisonments to a total of two years,[5] stated, “With our boxes of matches, and our necklaces, we shall liberate this country”, which was widely seen as an explicit endorsement of necklacing.[6][7] This caused the ANC to initially distance itself from her,[8] although she later took on a number of official positions within the party.[8]