The game could have separate message logs for user comments and game messages and always save the former in order to prevent the improvised journal from getting lost.

This seems a good time to dust off this suggestion and put it in the spotlight again. Since team ADOM blessed us with cinematographic landscapes and musical score, having to exit fullscreen and switch to some note-taking program shatters the immersion even more cruelly. So, even more than before, we need an ingame journal to write down the location of dungeon features such as altars and forges or other things we need to remember.
Earlier in this thread, TB pointed out that
I agree that we cannot risk a feature-creep escalation. However, we don't need a full-fledged editor. In fact, we don't even need an editor. We just need a way to add entries to a journal, display them, and mark them for removal. See for example the previous comments by Sami, snowball_ii, and Aielyn for some ideas along these lines. Here I'll add another example of how a simple journal could work; I took inspiration from the skills display screen and the 'Drop items in a comfortable way (Ctrl-d)' command, hopefully this will allow for some code reuse and make the journal easier to implement.

The journal would use two new keybindings, let's say 'j' and 'J': the first displays the journal, the second prompts the player to write some text.

Prompt
Suppose you gold-detected a royal vault in PC4, but you have no means of reaching it at the moment. Press 'J', and the game prompts you to write a note:
Write something, press 'Enter', and it will add the text to the journal. If you just press 'Enter' without writing anything, it does nothing. We could have a limit of 65 characters, which should suffice for most use cases and assures that each entry takes up a single line in the display screen.

Display and removal
To display the journal, press 'j':
Later in the game, when you don't need the note about the royal vault anymore, display the journal and press 'c' to mark the entry for removal; maybe it could display an 'x' on the left, like this:
and, when you display the journal again, the third entry has been removed.

Of course if the character is illiterate this feature would be unusable...

I think this is a journal the game keeps for the player, not the PC. Same like Monster Memory, the PC doesn't use that information, only the player. Who probably in most cases are literate. :p

I think the move number and also location of the entry should be automatically added to the log. E.g.

---------------------------------- JOURNAL ----------------------------------

A - M:124 L:BVL - 1st kill outlaw leader
B - M:1056 L:UD6 - white altar
C - M:12478 L:PC4 - royal vault in SE corner

[+] Page down --- [-] Page up --- [Z] Exit journal
[A-C] Mark an entry for removal

TDau: I made a post saying there's no reason not to simply use Notepad earlier on, but now that we have tiles and fullscreen, you make a valid point that switching back and forth to write and read notes is cumbersome.

I think your suggestion, with the addition IGB mentions, would be good - and hopefully simple enough to be implementable.

I am still playing ADOM 1.1.1 in DOS-Mode because it's so convenient
to use the mouse for marking text and copy to the editor I like to create my journal.

I know that there's a screenshot possibility
but then I have to handle 2 windows with 2 files next to the ADOM window,
one is the screenshot file and one is the Journal file.

I think we would'nt need an editor ingame,
if the Creator would find a way to mark and copy in the output window of ADOM,
but that's probably a hard task.

But even simplier would be a recommendation of a good screenshot-to-text tool
(so the creator doesnt have to do anything at all ... )
So does anybody here have made experiments with tools like

- ABBYY screenshot reader
- Able2Extract
- ...

or alike? Do they work with ADOM?

You are missing out on a lot of bug fixes and improvements. 1.2.0 prerelease 18 does have a dos version.

Since most of my chars die early, I am too lazy to write down where I encounter altars/shops, not to mention the first kill.
Needless to say, if one survives long enough, I start to regret about it =)

It would be great if major events were logged automatically.
Of course, the first kill should never be saved automatically... that's the cruel fun of it =)

Manual entries are undoubtedly necessary.