Is it worth buying 2021 iPad mini 6 now if it can’t get security updates in a year or 2?

Security updates most of the times keep longer then version updates. So that your iPad cannot update to iPadOS 18 doesn't mean that it gets no more security updates, it gets no more version updates. iPadOS 17 still gets security updates

But, I would advise against buying a 2021 iPad mini. It is almost 2025, that means that the 2021 iPad mini is almost 4 years old. In the times of recent tech evolution, that Is a lot. If you still have an older iPad, or Mac, you can continue to use it with no problems, but I would not spend hundreds of dollars/pounds/euros/any other currency on a 4 year old device

Given the concern you described, the only way to maximize the upgrade life is to buy new and Apple-direct when the device first releases.

Security updates end dates should be clearly identified during the purchase phase or clearly listed on their official website.

Apple will mot see your concern in these user-to-user forums. If you wish to place this before an Apple employee, I invite you to take advantage of this feedback link:

Feedback - iPad - Apple