Welcome to the site !!
I am guessing that it is frozen where it goes into the engine case , just had a issue like this with a buddys 02 CR250 , and we tried beating it , spray penatrents , and heat , and ended up having to cut it off carefully where the swingarm attaches , we used a wheel grinder with a very thin wheel so it would do minimal damage , you will need to remove the subframe to get it out of your way , removing the shock will be a must as well , you have to be patient if your going to do this , but once you get the swingarm off , then you can get the remnants out easier , most of it will fall out , it is possible you may have to beat it out of the case , then replace the collar that the bolt sits in in the engine case once you get everything separated , you will need a new swingarm bolt , and most likely a new collar
Unfortunately , when those things freeze in there , its tough getting them out , especially without damaging more stuff , this is why periodic maintenance includes greasing the swingarm bolt and linkages , just so you don't have issues like this , but unfortunately , most people skip this and this is the outcome , every winter the swingarm should come off and everything cleaned and greased with a waterproof grease (I use Maxima Blue Waterproof Grease) , it really sucks , but basically , blunt force , lubrication , and heat are about the only possible ways to get it out , if it will not come out with those methods , then cutting is your only alternative ....