A solo trombone in church music could be associated with the Trumpet of the
Last Judgment.  For instance, in the Requiem (Tuba mirum).

As for doubling of the choral parts, the trombones always doubled altos,
tenors, and basses.  Even though altos were probably boys' voices, doubling
them couldn't be attributed to shortage due to military activity.  Also,
this custom lasted for several decades, irrespective of whether a war was
raging or not.

-Margaret Mikulska