What is the historic accuracy of leather laced sleeves on a gambeson? I know they often had slits under the arm for ventilation, but a warrior is also unlimely to be stabbed there. For leather laced sleeves it creates an issue if the maille is breached. However, I see it implemented into a lot of gambesons available in stores and none of them really comment on this. Were leather laced sleeves really a thing or just a practical update to make gambesons more customizable for customers?
Mandatory "not a historian" preface, I am a historical reenactor that mainly focuses on the 11th-12th-13th centuries in Europe.
Historical sources on the gambeson/aketon are scarces to say the least, the most frequent being inventory lists such as this one (Richardson, The medieval inventories of the Tower armouries 1320–1410) that often give a great insight on the material but a rather poor one on the construction itself. That said, I am not aware of any historical example of a gambeson with removable sleeves. No manuscript miniature or surviving historical artifact that I know of show them, nor that I have ever read any written description of them. That makes citing academic sources quite difficult.
From a personal point of view, it does not make any real sense to make one. In the case of a gambeson that would be worn as the sole layer of protection (as were probably most of them in the early middle ages), as you pointed out it would make a realy obvious flaw in the defense of the arms/armpits (which is a rather critical area).
In the case of a gambeson worn under mail, from my personal experience they get really uncomfortable after a while. To be able to raise your arms properly, you need to give the lace some slack (at least at the bottom). At some point, that slack get on the top point of your shoulder (quite visible here), creating an unprotected zone where your mail really, really wants to bite.
Finally, in the case of a gambeson worn under full plate armor (it would probably be best refered to as an arming doublet, then), please bear in mind that the garment constitutes an integral part of the load-bearing capacity of the wearer. Armor parts (in this case, armor for the arms) could be directly attached by laces to the doublet sleeves or shoulder (see here for instance for an arming doublet with noticeable arm laces). It seems rather odd to me to break the continuity of the doublet by lacing the sleeves to the torso, thus putting some noticeable weight on the laces.
It doesn't seem sleeveless gambesons existed alltogether, with the notable exception of one being mentioned in the King's Mirror, altough it is descripted as an additional layer of protection on top of a hauberk and underlying (sleeved) gambeson.
