Sometimes. It depends on who owns it, and that is decided by your contracts. Most agency agreements assign the work to the client once it is paid for. If yours do, that code is not yours to license, however long it has sat on your servers. Some contracts keep the IP with the agency and give the client a license instead. Code built under those terms can qualify.
The easier wins are usually closer to home. Agencies build a lot of software for themselves: the CRM that ran the studio for six years, the deploy scripts, the reporting dashboards, the starter kit every project began from, the product you built between client jobs and then shelved. That code is yours, and it is exactly what AI labs want, because it was written by working developers under real deadlines.
If you are not sure who owns a repo, check the contract before you put it forward. You only submit code you hold the rights to, and you decide repo by repo.
Who owns code we built for a client?
Look for the IP or ownership clause in the contract or statement of work. If it assigns all work product to the client on payment, the client owns it. If it says the agency keeps ownership and grants the client a license, the agency can license it too. Many agencies also keep the rights to their own reusable components, even inside client projects, through a carve-out for pre-existing or general-purpose code. Where a repo mixes both, only the parts you own count, and it is usually simpler to leave that repo out. When in doubt, ask the client or a lawyer before you submit.