I'm aware, but since pretty much all trackers after ST3 supporting the S3M format (most notably Impulse Tracker) do support fine slides and some tracks make use of them, I was putting off implementing this quirk.

Do you know any ST3-made modules that require this quirk to be emulated in order to sound correctly?

I did find a few modules that seem to detect as made with ST3 that happened to use fine-slide K/L on ModLand (I have no idea how much to trust it though..it also doesn't show up that much from what I found, so its impact on playback might not even matter)

If OpenMPT says the file was saved with ST3, then it definitely was (the fingerprinting is pretty reliable that way around). Of course it's still theoretically possible that the file was composed in another tool and then re-saved in ST3, but usually it's the other way around (users upgrading to IT or ModPlug and resaving their old files).

Can you name some of the examples you found so I can check them out?

sure thing!

• KeyG/windows 3.1.s3m (pattern 6 and 7, row 37, channels 3 and 4)
• Klausi/there and back again(bilbo).s3m (pattern 6)
• Linearic/mirror of souls.s3m (patterns 3, 5, and 16)
• Manwe/no one can steal you.s3m (used throughout patterns 0 to 4)
• Resonatix/desert dream.s3m (patterns 7, 8, and 36)

Fixed in r21826. The new compatibility setting for this is only applied to ST3-made S3Ms. As compatibility flags are not stored in S3M files, this also means that resaving such a file in OpenMPT will cause fine slides to be interpreted again when the file is reopened.

Also documented at https://wiki.openmpt.org/Development:_Test_Cases/S3M#NoCombinedSlidesOnFirstTick-Fast.s3m