If the bottle is intact, the water is clear, and you’re not a germaphobe: go for it. Worst case, you’ll taste a faint plastic note and remember why you bought a reusable water bottle.
If you’re still nervous? Pour it on your plants. They’ll love the mildly plastic-flavored hydration. Or use it to flush a toilet. That’s what we do with old soda anyway.
Remember: water is the universe’s most durable liquid. It’s been through dinosaur stomachs, underground aquifers, and your own recycling bin. A few extra years in a bottle is nothing. Stay hydrated, you magnificent mammal.