I would caution against just throwing it in the trash.
I save all my old line and anything I pick up streamside and throw it in the firepit or burn barrel.
This way it turns into a plastic clump and I can sleep at night knowing I didn't throw out line that could possibly entangle wildlife, regardless of how remote it may seem.
Just my 2 cents...
In the "old days" when we had local bait & tackles most had big boxes provided by Berkley for old line. Fill up the box and ship it back to Berkley and they recycled it... supposedly not into "new" line but judging from various Trilene types I've tried I'm not sure about that ... 😀
As far as I know and have been told by several line makers..... Mono will last quite awhile as long as it is stored in a cool, dry place out of direct sunlight. I bought bulk spools of Silver Thread AN-40(6,10 + 17lb test) going on 3 years ago and it is just as good as the day I bought it. I've also got bulk/filler spools in when I worked at the shop that were bad right out the box. I don't know if it was a defective batch, sitting in a hot warehouse in Mexico or what.
The OP said the line was around 10 years old. He said it was kept in his garage and not exposed to the sun. 2-3 years, 4 at the outside, the line might be OK. But 10 years in the same position wound around a spool? No way it would be worth using.
He said he already pitched it. EightPoint had a good suggestion about the line.
I also remember seeing places to throw away old line in tackle shops a long time ago. I agree about Berkeley Trilene line, I don’t like it at all.