From Ubisoft: “2-Step Verification will soon be a mandatory requirement for PC players to play ranked in the APAC region. Ranked 2SV lock was previously announced for all other regions when it was first put into place, and this change will standardize the ranked 2SV requirement on our global servers. 2SV is not required for Console players in Ranked.”
Source: N4G PC Rainbow 6 Siege now requires 2 Step Verification for PC Players that play in APAC region