I looked at the "nm --dynamic"; I did see some dictionary API's:

  $ nm -D /usr/lib/i386-linux-gnu/libzstd.so.1.3.3 | grep -i -E 'init|create'
  [...]
  0000fa50 T ZSTD_createCDict
  0000d3a0 T ZSTD_createCDict_advanced
  0000fae0 T ZSTD_createCDict_byReference
  0000d6f0 T ZSTD_createCStream
  0000d6c0 T ZSTD_createCStream_advanced
  00042890 T ZSTD_createDCtx
  000427b0 T ZSTD_createDCtx_advanced
  00046a10 T ZSTD_createDDict
  00046960 T ZSTD_createDDict_advanced
  00046a40 T ZSTD_createDDict_byReference
  [...]