To whoever it might interest, how SK manages the one-way "so simply without difficult rule" ticket system, here are some observations i made recently trying to find on short notice a way to get to Paris from Copenhagen.

For a few dates, most SK flights were C9 Y9S9B9 E9M0H0... meaning IMO there was still quite many seats available in all classes but all of them proposed for quite a high fare (E = P-class service at very high price!) in order to compensate the tickets that have been bought 2 months in advance in TULK classes (i don't assume there are many of them). I looked at other destinations within a week and most flights were sold out in P-class (except E booking classes) and wide open in other classes.

AF still has a system with Economy ticket = non changeable, saturday night or 3 nights rule, return trip only. For intra Europe, the booking classes are K M Q W E N and for long haul, they have different buckets K H T V L (most expensive down to cheapest). On days when SK was in the C9Y9E9M0 configuration, AF had exactly the same! C9Y9K9M0 (Y is Tempo challenge, a bit like eco flex but with free champagne

... and at the same time they have T9 V9 L9! it would therefore be cheaper to go to New York than to Paris on these days, on the very same european flights! L9 proves that there are stil many seats available in eco.

Conclusion:
There is no advance purchase rule anymore in the scandinavian market but the cheap buckets seems to be chronologically closed in order to leave only expensive tickets available a few days before departure. as you know, i love how marketing tries to make us believe it is possible to buy 650 SEK ticket up to departure... AF seems to open all seats up to Q/W 48/24 hours before departure on quite many flights but not all (one can see K2M2Q2W2E0N0l on AF and i have never seen E2M2H2Q2V2W2L0 recently on SK)

it is getting more and more difficult to escape for a weekend somewhere ex-scandinavia with this fare structure while from France, where advance purchase applies everywhere, it is possible to find weekend fares, max 2 days, to go to Spain, Italy, germany, UK, so easily for around 150 EUR. And you get food and drinks.