As of March 2021, YouCompleteMe compiles fine on Termux, but it crashes once the plugin is loaded. The following error can be noticed in the logs:
ImportError: dlopen failed: cannot locate symbol "_ZNSt6__ndk14__fs10filesystem18__weakly_canonicalERKNS1_4pathEPNS_10error_codeE" referenced by "/data/data/com.termux/files/home/.vim/bundle/YouCompleteMe/third_party/ycmd/ycm_core.so"...