My iPhone 12 mini was bugging me about it the other day. I declined it. I don't want liquid glass and whatever else it does to make that phone feel slower and less usable. I refuse to buy a newer iPhone. They are all too big.

Wrong. Enable 18 beta, refresh, install 18.7.3, disable beta. Problem solved.

Security updates are typically available for the most current 2 OS versions, and 18 is still officially supported, perhaps until 2026 or 2027. 18.7.3 exists with similar security updates as 26.2. It may not show up on iPhone as an update option without being on the beta 18 channel because they're trying to force people onto 26 using dark patterns, but it shows up on iPadOS without any additional magic.