Unfortunately, MaGog doesn't have that information. All she keeps is:
1) date and discount of last sale
2) date and discount of best sale
If the two are the same, it's a historical max, but there's no way of knowing from these if it's a first time max or not.

It;s great!
Historical max = last discount > best discount sale. MaGog has this info and can compare, right?

And can you please consider some easier way to filter best sale price without adding a date of current sale, please?

Like this maybe:
Historical best = Past Sale - discount of best sale < last discount
Best sale price = Past Sale - discount of best sale <= last discount

What do you think?

I'm not sure you understood. MaGog does not maintain full history, it just maintains the four items I mentioned above.

Accordingly, last discount can never be larger than best discount. Because then the last discount is the best discount and the two are the same. MaGog does not maintain information about the previous best.

I can, however, add a more user-friendly filter ("last sale is/was the best"), which is:
(date of last sale == date of best sale) AND (discount of last sale == discount of best sale)
the second comparison is necessary because GOG might change its discount for a game twice in one day (not unheard of).

Thanks for the suggestion. I'll try to do it this weekend.

Thank you mrkgnao! Would be great to have "last sale is/was the best" option.

Now it works this way:
If last (current) sale = best sale that already has happened then MaGog will store actually only two items as "date and discount of last sale" = "date and discount of best sale", right?

Right.

Thank you.

Could you please check the following links (while logged in to GOG) and tell me what you see. If it's some text beginning with {"title":"Something Something Something", please copy paste the "Something Something Something" here. If it's empty or inaccessible (often redirecting you to the home page), please let me know:

https://www.gog.com/account/gameDetails/1104171850.json
https://www.gog.com/account/gameDetails/1128126797.json
https://www.gog.com/account/gameDetails/1231155730.json
https://www.gog.com/account/gameDetails/1444908480.json
https://www.gog.com/account/gameDetails/1459508003.json

All 5 of those links resolved to https://www.gog.com/##openlogin redirecting to the GOG homepage.

Great. This means these are obsolete IDs that are no longer relevant. I will let MaGog know that she can ignore these from now on.

Now that I think about it the IDs probably correspond to the Pre-Beam Dog, non-Enhanced versions of BG 1&2 and the pre-Bethesda Black Isle versions of Fallout 1, 2 & Tactics. Thanks for the hard work.

EDIT: At first thought that something wrong with [Age on Addition] as it should = full years( [Current date] - [Addition Date]).

But then figured out that [Age on Addition] = full years( [Addition Date] - [Release Date]) :-)

Does MaGog has any help page?

Exactly.

It's used primarily to determine if a game was "old" or "new" when added to GOG, for example on the Metrics page.

There is the TuTorial. :)

Done.

Works like a charm! Thanks a lot!