The logistics of combining fields in indexes and identifiers is relatively complex, while the logistics of indexing a single field is comparatively trivial. This is also why you don't ship timestamps using separate fields for second/minute/hour/day/month/year, but a single ISO-string or UNIX timestamp as representation: it makes querying and interpreting the value more consistent.
It is noy just the DDL of the primary table that needs to care about it, but also all foreign keys, DML, queries, APIs accessing the data, etc. Storing that UUIDv7 is likely going to be cheaper than pushing the cost of keeping a composite identity onto other components and systems that work with that data.
Well, that depends on how you manage it. An ORM will find it trivial. Custom SQL, not always so much.
That said, PostgreSQL offers https://www.postgresql.org/docs/current/rowtypes.html which gives you the best of both worlds. A single field which contains 2 fields that can later be extracted. So everywhere you can write a single field for joins, etc. But then when you need it broken out...
