If by "really M-Disc" you mean "substantially different from standard HTL BD-Rs", this has never been the case. See this review from when they first came out in 2015: https://goughlui.com/2015/10/16/review-tested-verbatim-lifet...

> While the immediate burn quality seems relatively satisfactory, I was still not satisfied as to whether these discs were truly worth their premium over commodity blank media. Looking at the discs, it seemed that the recording layers were likely made of similar materials based on the visual aspect, unlike the obvious difference with DVDs. Also, it seemed that regular recorders were intended to be able to record to the BD-R’s (according to the Verbatim text).

...

> Digging further around the M-disc site, it is noted that their section on longevity testing actually features no data about the BD-R discs at all. Indeed, if there were tests, the reports were not actually featured on the site at all. Further poking around in M-Disc Technology pages shows only references to their DVD products. In fact, it seems their patented “etched in stone” material may only apply to their DVD products.

...

> From my research into the data, it seems likely that the M-Disc BD-R is not substantially different from other quality BD-R discs on the market as they all use inorganic recording compounds (with the exception of BD-R LTH of course). From a visual inspection, the disc appears the same bright chocolate-brown, unlike the obvious distinction with regular dye based DVDs and the M-Disc DVD which was a silver-grey. But my main concern lies in the demonstrated lifetimes of quality BD-Rs which are similar to the M-Disc DVD (to which the M-Disc BD-R is claimed to have also achieved). In essence, it appears that the MKM/Verbatim BD-R DL has already reached the same level of endurance as the M-Disc DVD had claimed without being “special” at all. It is thus, hard to justify paying more for an M-Disc BD-R, and I cannot really recommend it.

The "not really M-Disc" stuff comes from a post somebody made on the "Data Hoarder" Reddit board a few years ago. The discs were changed to enable higher burning speeds which resulted in a slight difference in appearance, but he/she didn't seem to have realized that Blu-Ray M-Discs were never substantially different from BD-Rs and instead assumed that the tweaked discs were more of a scam than the originals.