The final version of libcelt is bit-for-bit compatible with Opus. Unlike SILK, there was never a "stable" version of CELT (we routinely broke the bitstream on almost every commit), so CELT development continued after the two codecs were merged, right up until the Opus bitstream was frozen.

That makes Opus a strict superset of CELT, and we've stopped maintaining libcelt, so any existing users should switch to libopus.