I believe flash can has the same criteria as a flash hider or any other muzzle device. As long as it’s unremovable (pinned&welded), it will count as the overall length of the barrel. If it’s removable (just screwed on with locktite), then it does not count towards the barrels overall length.

With that being said, I don’t foresee a need to ever remove a flashhider, flashcan or any other muzzle device other than an actual suppressor (mainly a .22lr one). Any cleaning you want to do to a muzzle device can always easily be done with it attached.