Patagonia boxers are made using recycled plastics and they also accept worn out boxers for recycling. Patagonia is the only boxers I have found that are very loose fitting (baggy in fact), silky feeling, yet stretchy, yet moisture-wicking all at once. Nothing like this seems to exist in Europe.
So here’s a debate: synthetic vs cotton
Synthetic boxers can be recycled and can be made from recycled plastics. But every time synthetic clothes get washed they shed microplastics which most sewage treatment centers cannot filter out. You would have to buy a special filter to attach to your washing machine. Researchers in Ghent discovered that the bacteria that loves perspiration also loves synthetic clothes but not cotton. This is why synthetic clothes get stinky fast and thus need more frequent washing than natural fibers.
Cotton production consumes absurd amounts of water (2700 liters of water to produce 1 t-shirt). And when you wash it, hang drying takes /days/ (whereas microfibers hang dry in a couple hours). So people use energy wasting tumble dryers when cleaning cotton. But cotton has the advantage of being biodegradable. You can simply compost/landfill finished cotton as long as it doesn’t have harmful dyes that leech out. There is also a cotton t-shirt that is claimed to wearable 7 times before each wash. IIRC it’s blended with silver for anti-microbial effects.
The environmental debate can go either way depending on which problem you want to focus on, but cotton is clearly lousy performing underwear considering how it retains water and gets soggy. The only natural fiber that performs well for underwear is wool (ideally Marino from what I’ve read). But the prices on that are extortionate. €60+ for one pair of wool boxers, and they’re tight fitting.
Anyway, the OP’s thesis is lost. There is no BifL boxers AFAIK.
There are BifL socks though, called “Darn Tough” which have a lifetime warranty. They have 1 competitor but I forgot the brand. Both use marino wool.
The answer is nearly always Natural Fibres for two reasons:
environmental - synthetic products do not degrade. Why wear something that literally microplastics everywhere you go and then gets thrown in a landfill at end of its use.
comfort - breathability is the key criteria for clothing. Polyester and synthetic fabrics are nearly all terrible at this compared to natural fibres.
Merino wool is one of the best products, especially for warmth. You don't have to pay Ice Breaker money, although it is becoming harder to find at affordable prices.
Linen is also a great fabric for warmer climates. Couldn't imagine a polyester t-shirt, let alone underwear, if I lived somewhere hot.
breathability is the key criteria for clothing. Polyester and synthetic fabrics are nearly all terrible at this compared to natural fibres.
Natural fibers cannot be grouped together in this way because there is a huge variation.
This is where cotton fails and synthetic microfibers come out ahead. Cotton retains water, swells when wet, and suffocates as water tension spans the threads that are thickened by the swelling. Synthetic microfibers wick moisture away, and do not swell when wet, which gives excellent breathability. Cotton is fine as long as you don’t sweat. Or exceptionally, if it’s extremely hot in some windy situations the water retention can be a plus. I used to don cotton and hose myself down before getting on a motorcycle on a hot dry day. The evaporative cooling effect worked wonders with the high relative wind. But outside of that niche, such as sports, microfibers are king which is why sporting goods shops fetch high prices for high tech synthetics. As someone who sweats profusely more than normal, cotton is a non-starter in warm climates. Evaporation from soggy cotton simply cannot keep up with the rate that I add sweat. So a cotton t-shirt gets soaked in sweat and remains wet the whole workout session, and for days thereafter.
I used to wear tighty whities which made my gear sweat. Switched to Pategonia boxers and wow what a difference in breathability.
Wool and synthetics are similar w.r.t. comfort hence the term “smart wool”. But indeed natural wool is pricey and non-vegan.
There are BifL socks though, called “Darn Tough” which have a lifetime warranty. They have 1 competitor but I forgot the brand. Both use marino wool.
I don't really think socks can be a BIFL item. Darn tough is definitely one of my favorites, but I've worn through pairs. Merino is a weak fiber (which is what makes it soft), so it wears out. I've got some pairs where the nylon structure of the sock is intact, but the wool itself has been abraded away. Yes, I could swap them for new ones, but I don't want to exploit a company who is trying to do things right.
There's a few companies making good, made in America wool socks. Farm to Feet, Camel City mill, grip6, a decent part of smartwool's portfolio, and probably a few others.
I guess the whole point isn't actually things that last forever, it's things that reach the optimum of longevity, eco friendliness, and performance.