The flag marks a point without doing anything with that point. Any other Most other save icons, including the curled arrow, again suggests writing to disk, which is sad. Also Clones used the flag for savestates. Not sure if more games do.
Granted I'm not much of a gamer nowadays, but from my limited, possibly very outdated experience, most games don't really offer savestates in the way Lix or even Clones does. RPGs often do, but they also tend to have an extensive text-based menu system to begin with, and it's easier to just put the options there in plain text without icons. In many other games (and also some RPGs too), save points are limited and are encountered within the level as some consistent but random object that generally stands out from the environment, but otherwise doesn't obviously scream "I'm a savepoint/checkpoint" on first glance. They may be mentioned in the game manual or some in-game tutorial.
Most emulators do have savestate features, but again, they also tend to just use a traditional text-based menu system rather than icons.
I don't remember exactly how I learned the UI of Clones, but I seem to recall it does do mouse-over tooltips, which if true is most likely how I learned. The flag is a reasonable icon, but I suspect it'll be tough to find an icon that most people will immediately recognize as savestate/loadstate--especially when many people may not even expect such a feature in a game--sure, we all can't live without it, but especially for new players, it is rather foreign and never present in any of the official Lemmings games.
Still, maybe the flag is good enough since at least it doesn't imply saving to disk.
If mouse-over tooltips won't be implemented in any form, maybe display some response text somewhere whenever user clicks on the savestate/loadstate icon? Saying something like "current state of level captured. Use the next button to the right to reload saved state" or similar.
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As for splat ruler, I actually think the double arrow that is also bookended by the lines is standard enough of a depiction of length/measurement, that I don't think it will be confused with resizing.
No plans to try to depict the "splat" part in the icon somehow? Or is that probably too difficult within the confines of the icon's size?