Training solo?

The tier 1 guilds (raider, enforcer,and fighter) have much less room for growth, especially if you have good stats.  Unless you really know what you are doing and are doing it pretty intensely, you might get stuck in jman weapon skills.

Why would using Byn for training be considered Meta? Merchant and Noble houses would sometimes pay Byn for their employees to join Byn strictly for training. Nobody in Byn "has" to join the contract. I mean you wont stay there long, if you're trained and useful enough, but clearly choosing not to help the team with dangerous contracts. But if it isnt too emotionally in your face, then why not? They're not paying you.

Is it not that training in the dark only trains blind fighting and nothing else?

All newbies get nudged firmly towards the Byn because it's a pretty good introduction to the coded aspects of the game, particularly combat and travel. The roleplay is usually trash tier but that's pretty much a given when you've got a clan full of newbies, which also means there are very low expectations of players in the Byn. You can easily get away with being offline for most contracts, especially if you aren't trying to carve out a future as sergeant. If you absolutely never participate in anything, they might not be interested in keeping you as a trooper, but I've only heard of that happening in cases where someone was barely active throughout the first year. Runners won't really get kicked out unless they break one of the very few real rules, or do something truly retarded like dickslapping a noble, but you could be denied the stripe if they barely know you after a year.

While the Byn is a good introduction to combat, it's not a particularly effective way to train beyond the point of 'okay.' Even if you don't have ambitions of actually maxing out your combat skills (almost nobody really does that), you'll still feel the effects of the combat code's shortcomings in the Byn. Unless the stars align for you and there's an elf heavy-combat character with high agility and solid skills who's around during your playtimes and prefers to spar with you over all the other dudes tugging at his sleeve for the same reason, which is a luxury I've never personally experienced (has there been a long-lived Byn elf since Sharak?), your combat skills - aside from parry and shield which are easily maxed - will stagnate around the midway point, aka the plateau.

If you're not trying to do something special like running a raiding crew or being the Guild hitman, you probably don't need to care about getting any better than the plateau. You'll be slaughtering animals with ease, capable of safely hunting anything except the actual megafauna (mekillots, silt horrors, etc.), and you'll trounce any new characters in a fight. But you'll be one of scores of plateaued fighters in the game and you're not going to impress the dozen or so characters knocking about at any given time whose players know how to defeat the plateau and actually build a powerful character. And those are typically the players with the heaviest involvement in those aspects of roleplay where you have a reason to care how good your character is at fighting, so you're very likely to cross paths with them if you're into plots, which I expect most players would say that they are.

The Byn is still likely to be the best clan for sparring in Allanak, though, just because the militia is basically the same except with a quarter of the players, the merchant houses no longer have hunting divisions, and the noble house guard clans have been closed indefinitely. So, if you want to pass the plateau and you aren't blessed with a buddy who plays a twinked-up fighter, you have to start learning some absurd strategies to do it the rogue way. I call it the rogue way because you'll have to do things that everyone knows you shouldn't do, staff will smack you in the back of the head if they see you do it, yet everyone knows that people do it.

I was actually gonna write a guide on training without sparring, but I haven't played since they added the "always a small chance to dodge" thing so I don't know how that might have affected everything.

I'm pretty sure the off/def relativity stuff doesn't reduce the chance to zero until you're already at the point where the target can't dodge you anyway. If something can dodge you, you can gain on it (or could, before they added that feature where there's always some small chance to dodge), but the actual chance will reflect how hard it is for the target to dodge you. If you're lucky to get one dodge at all, it's probably a very small chance to gain. I don't think it has fully rendered solo training futile, it has mostly just ruled out stuff like blindfighting against snakes all the way to master. We don't know what the numbers are, but I had a combat character after that feature was implemented and it wasn't really noticeable because I stopped missing anyway which means I was able to gain just fine up until that point. But yeah, it means you can't drag a tandu into some dark cave and molest it all the way to the skillcap. This change made it harder to solo train in the sense that the really cheap tricks won't work anymore, and some targets may offer a reduced chance to gain once misses become scarce against them. If something can dodge you enough to where it's even worth your time, it's got enough defense that it isn't a big concern. By all accounts, this feature specifically targets blindfighting against harmless targets because there's always a guaranteed chance to miss something you can't see no matter what your skills are. Any other consequences are probably incidental.