I agree with Arhu in that the game does offer more than you make out. For example, you gave definitive no's to these:
S7: your choices have consequences – no
S8: advancing in the story requires thought (=> e.g. irreversible choices, moral dilemma, riddles, …) – no
Combat can be avoided through sneaking or gameworld manipulation.
Combat can be avoided through dialogue.
Directional facing plays a role (=> e.g. more damage from behind, flanking)
Characters don't die immediately but can be revived during combat.
Your choices can have consequences. Since you are able to attack virtually all NPCs you can limit your quest options and even lose the game by making evil choices or unfortunate choices. It's not a major aspect of play, but it is there. So, wouldn't this also feed into S8 to an extent. Also, advancing isn't always cut and dried as the quest Kresselak gives you involves a mystery, for example. Defeating Poquelin is a puzzle during a battle. Again, it's not a major aspect, but there are elements there.
You can avoid battles by sneaking past creatures or using invisibility and combat can be avoided by dialogue, as I've already evidenced in another thread with you on the subject of this game, it just doesn't happen very often, but it is there, and more than once. A Rogue can increase their damage from attacking from behind afaik. Players don't fully die in combat, there are options to revive them if you meet the requirements (such as either cost or spell level).
(Story) You can find and recruit new party members
This is an interesting definition where IWD is concerned. While the game does not have NPCs joining your party at all, if you play with imagination you can create these kind of events at any moment, because in IWD you can delete (or have die) party members at any time and them immediately replace them by creating a new one without reloading the game. Feel you are struggling without a Rouge in the party? Simply delete your spare tank and create a new Rogue character, they will level up quite quick. And you can do this at any point in the game (just be careful not to delete a character you don't want to delete, lol), and few games offer this kind of new party member freedom. But, yes, none of it is strictly story-instigated.
This is very much a "how long is a piece of string?" topic. By comparison to the game's immediate peers, PS:T, BG2, NWN, Fallout, then, yes, if the shop only sold these kind of games then IWD would indeed be the 'Linear' descriptor and that is how I would describe it.
However, for academic analysis purposes, there are a vast number of games to compare IWD to and to dump it in the same category as, say, Aarklash: Legacy would be somewhat of a travesty.
By being experienced with the game you can skip a really quite large percentage of the game's contents and still complete the game. This suggests it's not entirely linear. In fact, practically every screen offers you 'optional' areas, some of which even experienced players don't even know exist (the most famous being the Goblins being eaten by the insects, insects eating Goblins screens). And also here the definition of 'secret areas' is somewhat evident as such things as the Holy Avenger might be considered a 'secret area' as a regular player would likely never find it unless told where to look.
So I agree with Arhu in that examples of many of the points can be found, it's just that the game doesn't particularly prioritise these factors. Compared to GTA V Icewind Dale is linear. Compared to Aarklash: Legacy IWD is positively sandbox.