Generally, when your non-Singapore Citizen employee ceases employment with you in Singapore, goes on an overseas posting or plans to leave Singapore for more than three months, you must notify IRAS at least one month in advance and withhold all monies due to the employee from the date you are aware of his impending cessation of employment or departure from Singapore. This process is known as tax clearance and it applies to all work pass holders including Personalised Employment Pass (PEP) holders.

摘自：
https://www.iras.gov.sg/irashome/Businesses/Employers/Tax-Clearance-for-Foreign-SPR-Employees/Tax-Clearance-for-Employees/

如果公司不hold你最后一个月薪水，假如人一去不回没交税，公司自己拿钱去补就是了。
正规做法会叫你填一封Letter of Undertaking 声明，是否永久离开。
作为个人只需‘You should check with your employer whether the Form IR21 has been filed.’和乖乖给钱。

再FYI一下
https://www.iras.gov.sg/irashome/Individuals/Foreigners/Your-Situation/Tax-Clearance-Process-for-Individuals--Non-Citizen-Employees-/