As base system imported LLVM openmp there's no reason to keep this port. Like devel/llvm* it doesn't have SOVERSION but as ABI remained compatible over the years it doesn't seem to be necessary, anyway. See review D19767 for the patch.
For ABI compat see https://abi-laboratory.pro/tracker/timeline/llvm_openmp/ In many cases even forward compatibility would work. OTOH, devel/llvm* don't use RPATH, so rtld often picks up libomp.so from the oldest version and no one complained so far. ;)
A commit references this bug: Author: jbeich Date: Wed May 8 16:06:04 UTC 2019 New revision: 501030 URL: https://svnweb.freebsd.org/changeset/ports/501030 Log: devel/openmp: phase out - Expire after the last version without /usr/lib/libomp.so - Drop SOVERSION for seamless transition (i.e., avoid conditionals) PR: 236907 Approved by: bapt (maintainer) Differential Revision: https://reviews.freebsd.org/D19767 Changes: head/astro/siril/Makefile head/biology/abyss/Makefile head/biology/py-fastTSNE/Makefile head/comms/wsjtx/Makefile head/converters/pdf2djvu/Makefile head/devel/openmp/Makefile head/devel/openmp/files/patch-libomptarget_src_CMakeLists.txt head/devel/openmp/files/patch-runtime_src_CMakeLists.txt head/devel/py-numba/Makefile head/finance/quantlib/Makefile head/french/aster/Makefile head/graphics/blender/Makefile head/graphics/darktable/Makefile head/graphics/gmic/Makefile head/graphics/gthumb/Makefile head/graphics/lightzone/Makefile head/graphics/mandelbulber/Makefile head/graphics/rawtherapee/Makefile head/graphics/tesseract/Makefile head/math/R/Makefile head/math/ambit/Makefile head/math/blaze/Makefile head/math/blazeiterative/Makefile head/math/combblas/Makefile head/math/dbcsr/Makefile head/math/gretl/Makefile head/math/hipmcl/Makefile head/math/hmat-oss/Makefile head/math/libnormaliz/Makefile head/math/libxsmm/Makefile head/math/lis/Makefile head/math/mkl-dnn/Makefile head/math/py-rpy2/Makefile head/math/py-yt/Makefile head/math/timbl/Makefile head/math/tmv/Makefile head/misc/urh/Makefile head/science/abinit/Makefile head/science/chrono/Makefile head/science/code_saturne/Makefile head/science/dftbplus/Makefile head/science/gabedit/Makefile head/science/gromacs/Makefile head/science/lammps/Makefile head/science/latte/Makefile head/science/openmx/Makefile head/science/py-GPy/Makefile head/science/py-PyFR/Makefile head/science/py-phono3py/Makefile head/science/qmcpack/Makefile head/textproc/libfolia/Makefile head/textproc/ucto/Makefile