Wondering if this crankset from 1990 is still safe to use

As posted, I'd say NO, not safe to ride.
^{Hard to know from photos - my answers here might change if I saw them in person.}

Photo #3 looks easiest to test, by securing the crank arm in a vise and gently lever on the inside of the square drive hole with a screwdriver or something wooden. Don't go crazy with pressure, you're just looking to see if the crack widens under load. If it cracks further, then it was already dickered and you didn't break it any worse than it already was.

Photo #2 suggests a curved crack - this could be tested by threading in a pedal and leaning on that, with crank arm in a vise.

The crack in photo #1 looks like its aiming at the corner of the square taper, which would be the natural stress riser, but its not starting at the corner, so that one is confusing.

In #4, there's two candidates, but neither are where one might expect stress to accumulate, at the narrowest parts. Not sure on this one.

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A FC-M550 is a Shimano Deore LX crankset and dates from 1990 to perhaps 1992. Nominally it was a mountain bike groupset according to http://velobase.com/ViewComponent.aspx?ID=33595b3b-b98d-498e-b073-920e30da9748&Enum=115

Given the crankset is 30 years old, and its done a solid amount of work, I'd suggest replacing the crankarms and therefore the spider too. Your chainrings are probably 110 mm and 74mm BCD, and could be moved to a less-cracked chainset if they have wear left in them.

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What's the risk of riding these cranks as pictured?

Taking photo #3 - there are cracks radiating from the bottom bracket interface. So the most stress on this point is when your left foot is forward and you're powering into a climb, pass, or sprint.

When one cracked side lets go, the other crack will immediately fail too. The crank arm remains will be held on by whatever friction the bolt can give, which is orders of magnitude less.

Your left pedal would hit the road surface before you can react.

If you're out of the saddle, you will hit the saddle about the same time, or maybe miss it on the way down to the top tube.

Your left foot will now be going backward fast, and depending on your reaction time and where the front wheel was, you could fall to the left or to the right equally. So you will loose speed and skin fast.

In a bunch or pace line you risk taking out every rider in a cone-shadow behind you as they all evade or ride over you.
If you're riding in traffic, that could be a car right behind you instead.

Even if you're relatively unscathed (say you were just pulling away) then your bike is damaged and maybe only pedallable by one leg, and only if you have cleats/toestraps. Otherwise its a walk home, maybe a scootering on the other pedal, or perhaps an inelegant dandyhorse stride while sitting on the saddle. Perhaps its the dreaded phonecall of shame to someone at home for a pickup, or maybe someone else has to call because you can't.

Replace the cranks, as soon as you reasonably can. If you legitimately have to ride this bike somewhere, then take your time to stay seated and avoid any power efforts or climbs.

And remember it is totally okay to own more than one bike, for situations exactly like this.