Not sure if retro is proper place... I have two old notebooks with x86 processors in the 100s range of MHz. While nostalgics might appreciate them, I'm wondering if there is any proper point in keeping them. The hardware is modern enough such that to the best of my knowledge nothing would exclusively run on them but not on a current PC, whereas using them for modern applications sounds like a horrible idea in terms of energy efficiency. But just disposing of them (properly, of course) also sounds like pity... So what are, objectively, the best options?
The hardware is modern enough such that to the best of my knowledge nothing would exclusively run on them but not on a current PC
This statement partially answers your question, given that the idea of a "current PC" isn't frozen in time. 10 or 20 years hence, you may find it much more difficult to find a "current PC" that can utilize the old software (or, more importantly, your data) that worked perfectly well on that old hardware you decided to recycle 10 years earlier.
Using them for modern applications sounds like a horrible idea in terms of energy efficiency
A computer, like any tool, always works best when used as it was intended. So you have a very valid point here that old hardware that wasn't designed with the latest version of Windows or some application in mind won't work to the same level as the new hardware that was designed for the latest OS and applications.
Conversely (to use an analogy), I still have the same hammer in my toolbox that I bought 30 years ago and I've found that the nails I want to drive haven't really changed.