Can I restrct MATNR to 18 characters in S4HANA

I am migrating my ECC system to S4HANA and I am aware that the Material Length Extension Functionality Adoption is optional.

1)But if I do not adopt the functionality, will MATNR remain 18 characters or it will still be 40 characters ?
2) Will I need to adopt my Custom Code and BAPIs related to MATNR.