gen 5 are new, the others have been there forever. The issues with glocks is 940 CMR 16 where the AG has determined it is an unfair or deceptive consumer practice to sell a gun that does not contain either a loaded chamber indicator or a magazine disconnect. The glock does not have a magazine disconnect and while the LCI is no worse than the LCI on any number of guns that can legally be sold, the AG has determined that the glock LCI is not sufficient.
Then again, if you are not a dealer, none of this matters to you. Find someone who will sell you what you want and buy it. There are no restrictions on you as the purchaser other than you should not purchase an assault weapon.