ProjectSAM is legendary for their "Symphobia" series. "The Free Orchestra" is a curated selection of their best one-shot samples.
This is arguably the most famous free orchestral plugin in the world. The "Discover" edition includes a dedicated Trombone section and a Bass Trombone. trombone vst free
Orchestral Tools is a giant in the scoring world (known for the Berlin Series). They released "Layers" as a free orchestral library, and tucked inside the "Woodwinds" and "Brass" sections are usable trombone samples. ProjectSAM is legendary for their "Symphobia" series