It's a Timescale feature that allows you to "just insert" metrics without first doing a schema migration to make sure a possible new label schema is supported. I.e. a feature that is taken for granted in native timeseries databases, which don't have to work around RDBMS schemas.

I assume it's doing an automatic ALTER TABLE when necessary, which modifies each row and somehow breaks compression across the sharded tables. Or at least an automatic re-compression would cause massive latency on insert that they wanted to avoid.