I am just curious what methods have been found to softlock this game.

Here are some notes about what I am looking for:
1. A softlock is any situation where progressing through the main quest is no longer possible. This can include things like the game not accepting input or an important character getting killed. It does not include situations that prevent a sidequest from being completed. For purposes of this topic, Watcher's Keep counts as a main quest as does the area from the Baldur's Gate 1 expansion.
2. The softlock must be possible without hacks, mods or cheats. Glitches and engine exploits are acceptable.
2a. TuTu and Trilogy are mods and are therefore disallowed. Softlocks for the original Baldur's Gate must be possible in the original engine.
2b. Fixpacks also count as mods so don't post softlocks that require them.

Anyway, here is one that I can think of:
Start Baldur's Gate 2's tutorial and kill the tuturial guy. (Note that actually doing this is tricky; the one method I've found is to import a Throne of Bhaal character who knows Greater Deathblow.) Without the tutorial guy, there is no way to finish the tutorial and start the main game. (Well, you could still export the main character and import her into the main game, so this isn't a major issue.)

Walk into Irenicus' room in Spellhold, the one where Dradeel can sometimes be found, immediately after exiting Bodhi's hunt maze. Game over.

First of all, just to clarify something, a sudden game over is not a softlock. A good rule of thumb is that it is a softlock if you're stuck and one of the following is true:
1. You can save the game
2. You can't quit the game (without something like Alt+F4 or Task Manager)

I can think of a few more examples that I have read about:

1. In Baldur's Gate 1, there is an event where you are poisoned and must solve a quest within 10 days or die. If you wait until the time is almost up and save too far from where you need to go, you are stuck and can no longer continue on that save.

2. In BG2, wait too long to take Bodhi's side, and then kill the Shadow Thieves. You will need to deal with Arkanis Gath; Death Ward will prevent him from instantly killing you (with no save), and Shapechanging into a Mind Flayer will allow you to kill him despite his minimum HP item.

3. (Seen reported on the beamdog forums:
https://forums.beamdog.com/discussion/41386/how-to-kill-arkanis-gath) In the Enhanced Edition, anger the thieves in the Thieves' Guild and then escape from Arkanis Goth. Then get Bodhi's quest to kill the "shadow master". You will be stuck since the Enhanced Edition actually closed the stat drain loophole making him literally unkillable. (What is especially bad is that the Ctrl-Y cheat apparently doesn't work here.)

Edit: In general, killing important characters who have HP1 items (to try to prevent you from doing so) can often cause a softlock. Killing Imoen in Irenicus's dungeon will not; the cutscene when you leave plays out normally (with a living Imoen) and the dead Imoen is still removed from your party.

I know the one in Baldur's Gate I in Candlekeep. If you will start killing people in there and then save, the game has become unwinnable. The moment you come to Gorion, he will 1-shot you.

You can also kill innocent people in Athkatla in BG 2, and can attempt to hide from Amnish Legion in some house. If you save after that, you will be sooooo screwed (unless your characters are very high level).

Perhaps some patch fixed that, but I doubt so. Apparently there is one and only way to play through this game - by being good, even despite the choices being done in the dreams.

When playing Baldur's Gate for the first time my party managed to kill Marek after he gave his warning and was supposed to run away.
I continued for many days not knowing the game had become unwinnable, all progress was lost. Luckily I still had a save from just before entering Baldur's Gate city.