The GFortran intrinsic procedure docs are not up to date. They still use the d suffix for double-precision procedures, which are obsolete. This could be misleading because I saw people using these suffixes on stack overflow questions. ACOS (The GNU Fortran Compiler)
I don’t see anything wrong with the gfortran documentation. The page for ACOSD states clearly “This function is for compatibility only and should be avoided in favor of standard constructs wherever possible.”
Under the “standard” section, it’s also marked as a GNU extension.