Hello Sunny,
From technical point of view, I could share following information with you:
1. attribute of field "tax classifications" in customer master data(KNVI-TAXKD) => can be changed in transaction OVT0;
2. attribute of field "tax classifications" in material master data(MG03STEUER-TAXKM) => can be changed in transaction OMSR.
From business requirement pointof view, I am not sure in which situation it is necessary to change these fields to opitional. As mentioned by Veselina Peykova, country tax legislation is a very sensitive topic. Therefore, please consider your business requirement fully and test it carefully before changing
this customizing.
Please let me know if this is helpful or not.
Best regards,
Crystal Lin