It is hard to see that holding any water given that at least two minor clans were wiped out as soon as the concept was initially introduced to the game-- the Rabbit and the Snake, both of which eventually came back in some way because well... once something has a name, you can't keep it dead.
In both cases the Great Clan just said "they were using maho" and were taken at their word with no real investigation or challenge-- because testimony is considered better than physical evidence and conjecture.
So no sooner was the concept of Minor Clans introduced to the setting, but the precedent was set that a Great Clan was free to wipe out any Minor Clan without any attempt for the Imperial Legions to intervene (not that it would ever be worth it for them to intervene) or punishment-- just so long as they bothered coming up with and presenting a stock excuse for their actions.
And soon after that, the "Falcon Clan" was absorbed into the Crab Clan, thus setting the precedent that it is completely normal and acceptable for minor clans to just join Great Clans in order to avoid military action being taken against them.
If the Falcon clan can just join the Crab Clan on a whim, why wouldn't the Sparrow Clan just rejoin the Crane Clan or the Rabbit Clan join either the Lion or the Scorpion (whichever would have them)?
And the Dragonfly clan origin particularly stands out as odd given it was a big mess over an inter-clan marriage, something that as the setting developed was more and more indicated to be something pretty routine even between clans that weren't getting along, and ended with them being saved by the Dragon Clan which, once the concept of vassal families was introduced into the setting, is a pretty typical story of how a vassal family gets formed. Since the Imperials had nothing to do with the whole thing, the idea that this would have caused them to become a minor clan just doesn't fit. Nor was their stated role as they were basically just an extension of the Dragon Clan, serving as their envoys to the outside world as they generally kept to themselves.
And most certainly it is odd that the Fox never just rejoined the Unicorn. Instead somehow they ended up part of the Mantis Clan who lived half-way across the empire from them. As though any communication between them would even be possible, let alone collusion.
So if there is a decree, it is an unenforced one that clearly no one takes seriously. Or rather, it was just put there was an excuse once it was clear that the writers had written themselves into a corner, but completely forgotten about whenever such a decree would be inconvenient to the story they wanted to tell.
But, if one is going to reboot the whole setting, then the whole issue can be neatly avoided. Instead of devoting tons of effort into fleshing out 20 random minor clans each with their unique special snowflake abilities that no one else can do, that effort can be put towards developing existing clans.
I don't have an issue with the Mantis, because the Mantis are way out there on their little island and even if someone were to get it into their head to try to invade them, they would be putting their relatively untrained and untested military against people who practically live on boats. And even if they could win the war, governorship of those islands likely wouldn't be worth the cost of taking them.
But the Wasp and the Centipede are some really terribly written Minor Clans that lay bare all the flaws and faults with the concept and there has to be a better way to bring those concepts into the story if there is going to be a playable Mantis Clan in this itteration of the game.