Other elections with similar values tend to be small island nations with FPTP: Jamaica (1997): 25 Antigua and Barbuda (2014): 24.9 Bahamas (2012): 24.3

The combination of "high effective number of vote-winning parties" and "high disproportionality" is really rare. You're mostly looking at cases like Poland 1993 (25.6), where, in a new political environment, there was a huge tail of small parties which didn't win seats