is this highway definitely closed? and if so, is access physically blocked?

unknown

The speed limit for High Road is 70km/hr from Leach Highway to Nicholson Read, with a possible 40km/hr zone during school arrival & departure (between Meadowbrook Drive & Belvedaire Way)
School zone
40 kph on school days
7:30–9am and 2:30–4pm (or as per signage)
https://www.police.wa.gov.au/Traffic/Road-safety/Road-Safety-Information

This area should be a small park with a footpath through the middle

wikidata suggests, that there is Arthur River here. Maybe the Blackwood River is badly named. Please check.

this is a private driveway by the looks of it

deleted

This area is Sid Eaton Reserve

"closed down"
The place has gone or never existed. This is an auto-generated note from MAPS.ME application: a user reports a POI that is visible on a map (which can be outdated), but cannot be found on the ground.
POI name: eleven squares
POI types: amenity-cafe
OSM data version: 2017-10-20T12:20:02Z
#mapsme