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Regarding shelf life of batteries, alkaline cells typically have an advertised shelf life of 7 years from production, or about 6 years from purchase unless you get them very fresh.

Zinc carbon cells are usually about 3 or 4 years from production or about 2 or 3 years from purchase.

Multi-cell batteries sometimes have a shorter advertised shelf life than single cells. This is because with say 6 cells in series, there is about 6 times the risk of one being defective, if compared to single cells.
Advertising a shorter shelf life counteracts this since fault rates are partly related to storage time.

Consider the following example.
7 years after production, one cell in 100 is defective, perhaps acceptable.
If the same cells are instead assembled into 9 volt batteries, then after 7 years storage about 6 batteries in 100 will be defective, probably too high.
So instead the manufacturer might offer the 9 volt batteries with only a 5 year shelf life.
After only 5 years, the failure rate of the 9 volt batteries might be an acceptable one in a hundred, or about the same as single cells after another 2 years.