I tried to set the "max viewable distance" to 8192 and Half-Life 1 would just not respect it in-game. After using mutliple values, looking for solutions in the internet I came to one very subtle difference in the way Sledge exports the map (stored in SMF-format) to the old ".map" format comparing to VHE (used an old map for comparison):
Sledge:
"classname" "worldspawn"
"spawnflags" "0"
"mapversion" "220"
"maxrange" "8192"
VHE:
"classname" "worldspawn"
"spawnflags" "0"
"mapversion" "220"
"MaxRange" "8192"
Note the case of "MaxRange" is different in Sledge.
Then I just edited my .map-version with Notepad++, changed "maxrange" to "MaxRange", compiled it via Sledge and it worked just fine.
Apparently I was unable to find the place where this is set in the source code - in that case I would have created a pull request.
Is anybody able to confirm this?