API v5 will remain part of the product beyond the announced removal of v3, while REST API v6 is already available. For existing integrations, staying on API v5 is ok as long as no new features of v6 are required, but our recommendation is to start early to migrate to API v6 even in consideration of the effort, to be prepared for the situation when you need a new functionality that is available on API v6 only.
Further, following documentation referring to the api v5 is outdated!
For new customers, and also for new projects at existing customers, we strongly recommend to start with REST API v6. Please also see Migrate REST API clients from v5 to v6