Look into the 1284x key - it's used by the majority of police cars in the US , and you can easily find it on ebay for super cheap. Once you get one, you can get it copied at a hardware store and hand them out to comrades before protests.
Here's a talk discussing things that are commonly keyed, it's very good and definitely worth your time to watch https://www.youtube.com/watch?v=a9b9IYqsb_U (I was planning to make this post today but u/alfrednobel already posted the same talk lol, guess we had the same idea)
Other keys to look into would be the ones to doorking systems for apartment buildings - those boxes that let you buzz in to go inside. They have common maintenance keys, and with that key and a piece of wire you can let yourself into the building if you need to hide out for a while. These are the 16120 and 222343 keys. Again, watch the talk, it goes into this.
Stay safe comrades!
Did you also see the post about disabling bulldozers? That's the main interesting point I found in the video. You can order a key for a bulldozer for about the same.
I completely forgot about that part of the talk, lmao
I wouldn't see that being super useful at a protest, especially since you'd need to carry a large ring of keys to make sure you had the right one for any equipment you come across, but driving a backhoe or bulldozer through a line of cops sounds fucking hilarious.
I figure that if you know the general area you can check out construction sites beforehand. If you are going to take one you might want to learn a bit about how to drive them first as well. having one key on a key ring will get overlooked and leave you the option. Most of this is stuff that would only be needed if things get a bit hotter than they are now though.