Some people, schools, and other organizations held on to their eclipse glasses from the August 2017 Great North American Eclipse. That's great! I wish I still had mine for sentimental reasons.
The thing is, these glasses use a specialized solar filter made of silver polymer. That substance does not last forever. Like all things, it degrades over time. The current prevailing recommendation, and the obsolescence period that Eclipse Glasses USA has likewise adopted, is to discard eclipse glasses after 3 years.
Is it safe to use those old eclipse glasses? There's two ways to find out: 1) you send the glasses to a US lab for ISO 12312-2 testing. The glasses will be destroyed in the process but hey, you'll get a result?
The other option is to, well, put them on and look at the Sun. If you can still read this blog post after the fact, then I guess they're ok. Legal disclaimer: Eclipse Glasses USA is not endorsing observing the Sun with expired eclipse glasses. This was a comment made in jest only and is not to be misconstrued as official advice.