Sorry, not understanding the question. It is possible to create a many-to-many tables in Service (see the link).
ServiceNow uses MySQL as a database. It's a relational database. As such, it's not possible to physicallyt many-to-many relationship in MySQL.
However, ServiceNow's table aren't direct MySQL tables but logical tables. As such, it's possible to create a many-to-many tables.
The use of many-to-many tables depends on the use case. Whether it's a good practice or not will depends on how it is being used rather than the technology itself.
FYI:
https://servicenowguru.com/system-definition/relationships/creating-manytomany-relationship-servicen...
