A recent post on NASASpaceflight.com provides backup for my earlier comment
It's not the propellant that expires IIRC. It's things like the O-rings, hydrazine sitting in the TVC system, etc. Certification limits that may or may not be real, just what has been tested.
This article talks about the limit being seal-related.
“Where the stack life comes in is there are some features in the booster that move with time and gravity loads that are temperature dependent [for] how much they move. And we do some rather detailed analysis as to whether, for example, our field joints, where we have some sealing features and thermal barriers in those positions where how they mate together matters to their performance. And over time, their interfaces can change.”
This originally led to a one-year stack life, but that didn’t mean the SRBs were only good for one year, just that the initial certification went out one year.
“So as Artemis I was delayed due to various different issues, we were able to go show through some additional data that we had, some additional testing, and some additional analysis that we could extend that well beyond one year,” noted Tobias.
“And the boosters, from a stack life standpoint, were certified to go through the end of the year. And there were additional analyses in work that would have shown that they were good to go into the early part of next year as well.”