Got this cyclocross frame with a fork, didn't see the cut in the steerer tube until much later. Has a 1mm? deep cut on half of the steerer tube. Is this fork done for? Or am I just paranoid. Aluminum steerer, I think its from someone who tried cutting it but didn't go through fully. The cut isn't anywhere near the headset bearings but it is inside the headtube
Cause I would have called this a witness mark from rubbing on some kind of protrusion. If it were a cut, it would be narrower and concentrated in one place. Also notice the tiny swarf pushed out of the channel, cutting does not do that. I think this fork was wearing on something, possibly while installed in another bike.
Is it safe to ride? Hard to say - depends how much of the metal is left inside the steerer.
Repair take it somewhere that does aluminium welding and get their professional opinion on whether its weldable. Welding aluminium requires much skill and an AC TIG welder, its not something you can do with cheap home welders. Plus the weld may need to be turned down to get through the frame or lower bearings. You could potentially drop a sleeve inside the fork's steerer, and then pin it in place, but that would be a complex repair.
There are many things on a bike where some minor damage can be okay, provided you can inspect it frequently and reevaluate if the conditions change. Dented frame tubes for example. In this case, the damage will be inside your head tube and impossible to inspect easily.
Risk is that steering will go mushy and then completely fail as the steerer tube suddenly shears/tears. If you're riding at any speed, a fall is inevitable. Your bars will likely stay on the bike, but the front wheel would be uncontrolled and will turn sideways immediately resulting in a front wheel washout or a complete OTB. Either way, it would be sudden and injury-inducing. At the wrong moment, could easily see a fatality resulting.
ANSWER The correct answer is to replace the fork, shop for a replacement fork as soon as possible.
If you (as a competent adult) choose to take the risk and ride this, then disassemble the headset every month or two and inspect and photograph. After 6 months of no changes, you might lengthen the period of inspection to 3 monthly.
If the steering ever feels "weird" then stop immediately and stand in front with your wheel between your knees, and try to turn the bars while restraining front wheel. They should not move separately at all - if they do you're walking home, and can think how close you came to a bad accident.
If you sell the bike, ethically you should show the buyer the problem, and this question.
I have the same on my own bike, it has likely been there a while though I have only just noticed it. I don't feel it is too unsafe to ride but that is my own assessment (I don't ride competitively or particularly fast.).
I realised after a little thought that the cause of the 'cut' was simply using an exposed length of chain, with a padlock, as a bike lock for several years, keeping the chain wrapped around the frame at that spot when riding. Steel chain links against aluminium is bound to wear.
That's a safety problem.
Traditionally steerer tubes were made from steel and were 1". However, to improve the safety and durability, the size of steel steerer tubes was increased to 1+1/8".
Today we quite unfortunately see steerer tubes being made from aluminum or carbon fiber. Aluminum has a fatigue problem (use it for over 100 000 km and it could fail), carbon fiber has a problem that if the bike is in a crash, the forces could be so high that they cause invisible damage to the carbon fiber parts, and the parts then could subsequently fail "just riding along" with no warning.
I would be very hesitant to use an aluminum steerer tube in a bike that's ridden a lot. Perhaps a bike that sees very little riding and has 1.5" diameter of the steerer at the lower end and a riding position that places little weight on the handlebars might be acceptable to have an aluminum steerer tube.
This cut acts as a stress riser, especially because it seems to have a sharp edge. Heavy use over long periods of time causes a crack to develop, and the crack propagates through the material. Maybe you could use it temporarily while waiting for a replacement to be shipped, but for permanent use it's not good unless you inspect it after every ride. That will require so much work (because it a built bike the cut is not exposed) that I don't think it makes sense.